bazel: enable the full Windows gnullvm CI path (#15952)

## Why

This PR is the current, consolidated follow-up to the earlier Windows
Bazel attempt in #11229. The goal is no longer just to get a tiny
Windows smoke job limping along: it is to make the ordinary Bazel CI
path usable on `windows-latest` for `x86_64-pc-windows-gnullvm`, with
the same broad `//...` test shape that macOS and Linux already use.

The earlier smoke-list version of this work was useful as a foothold,
but it was not a good long-term landing point. Windows Bazel kept
surfacing real issues outside that allowlist:

- GitHub's Windows runner exposed runfiles-manifest bugs such as
`FINDSTR: Cannot open D:MANIFEST`, which broke Bazel test launchers even
when the manifest file existed.
- `rules_rs`, `rules_rust`, LLVM extraction, and Abseil still needed
`windows-gnullvm`-specific fixes for our hermetic toolchain.
- the V8 path needed more work than just turning the Windows matrix
entry back on: `rusty_v8` does not ship Windows GNU artifacts in the
same shape we need, and Bazel's in-tree V8 build needed a set of Windows
GNU portability fixes.

Windows performance pressure also pushed this toward a full solution
instead of a permanent smoke suite. During this investigation we hit
targets such as `//codex-rs/shell-command:shell-command-unit-tests` that
were much more expensive on Windows because they repeatedly spawn real
PowerShell parsers (see #16057 for one concrete example of that
pressure). That made it much more valuable to get the real Windows Bazel
path working than to keep iterating on a narrowly curated subset.

The net result is that this PR now aims for the same CI contract on
Windows that we already expect elsewhere: keep standalone
`//third_party/v8:all` out of the ordinary Bazel lane, but allow V8
consumers under `//codex-rs/...` to build and test transitively through
`//...`.

## What Changed

### CI and workflow wiring

- re-enable the `windows-latest` / `x86_64-pc-windows-gnullvm` Bazel
matrix entry in `.github/workflows/bazel.yml`
- move the Windows Bazel output root to `D:\b` and enable `git config
--global core.longpaths true` in
`.github/actions/setup-bazel-ci/action.yml`
- keep the ordinary Bazel target set on Windows aligned with macOS and
Linux by running `//...` while excluding only standalone
`//third_party/v8:all` targets from the normal lane

### Toolchain and module support for `windows-gnullvm`

- patch `rules_rs` so `windows-gnullvm` is modeled as a distinct Windows
exec/toolchain platform instead of collapsing into the generic Windows
shape
- patch `rules_rust` build-script environment handling so llvm-mingw
build-script probes do not inherit unsupported `-fstack-protector*`
flags
- patch the LLVM module archive so it extracts cleanly on Windows and
provides the MinGW libraries this toolchain needs
- patch Abseil so its thread-local identity path matches the hermetic
`windows-gnullvm` toolchain instead of taking an incompatible MinGW
pthread path
- keep both MSVC and GNU Windows targets in the generated Cargo metadata
because the current V8 release-asset story still uses MSVC-shaped names
in some places while the Bazel build targets the GNU ABI

### Windows test-launch and binary-behavior fixes

- update `workspace_root_test_launcher.bat.tpl` to read the runfiles
manifest directly instead of shelling out to `findstr`, which was the
source of the `D:MANIFEST` failures on the GitHub Windows runner
- thread a larger Windows GNU stack reserve through `defs.bzl` so
Bazel-built binaries that pull in V8 behave correctly both under normal
builds and under `bazel test`
- remove the no-longer-needed Windows bootstrap sh-toolchain override
from `.bazelrc`

### V8 / `rusty_v8` Windows GNU support

- export and apply the new Windows GNU patch set from
`patches/BUILD.bazel` / `MODULE.bazel`
- patch the V8 module/rules/source layers so the in-tree V8 build can
produce Windows GNU archives under Bazel
- teach `third_party/v8/BUILD.bazel` to build Windows GNU static
archives in-tree instead of aliasing them to the MSVC prebuilts
- reuse the Linux release binding for the experimental Windows GNU path
where `rusty_v8` does not currently publish a Windows GNU binding
artifact

## Testing

- the primary end-to-end validation for this work is the `Bazel`
workflow plus `v8-canary`, since the hard parts are Windows-specific and
depend on real GitHub runner behavior
- before consolidation back onto this PR, the same net change passed the
full Bazel matrix in [run
23675590471](https://github.com/openai/codex/actions/runs/23675590471)
and passed `v8-canary` in [run
23675590453](https://github.com/openai/codex/actions/runs/23675590453)
- those successful runs included the `windows-latest` /
`x86_64-pc-windows-gnullvm` Bazel job with the ordinary `//...` path,
not the earlier Windows smoke allowlist

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/15952).
* #16067
* __->__ #15952

.github/workflows/bazel.yml

@@ -17,6 +17,7 @@ concurrency:
   cancel-in-progress: ${{ github.ref_name != 'main' }}
 jobs:
   test:
+    timeout-minutes: 30
     strategy:
       fail-fast: false
       matrix:
@@ -39,9 +40,9 @@ jobs:
           # - os: ubuntu-24.04-arm
           #   target: aarch64-unknown-linux-gnu
 
-          # TODO: Enable Windows once we fix the toolchain issues there.
-          #- os: windows-latest
-          #  target: x86_64-pc-windows-gnullvm
+          # Windows
+          - os: windows-latest
+            target: x86_64-pc-windows-gnullvm
     runs-on: ${{ matrix.os }}
 
     # Configure a human readable name for each job
@@ -67,8 +68,14 @@ jobs:
           BUILDBUDDY_API_KEY: ${{ secrets.BUILDBUDDY_API_KEY }}
         shell: bash
         run: |
-          # Keep V8 out of the ordinary Bazel CI path. Only the dedicated
-          # canary and release workflows should build `third_party/v8`.
+          bazel_targets=(
+            //...
+            # Keep standalone V8 library targets out of the ordinary Bazel CI
+            # path. V8 consumers under `//codex-rs/...` still participate
+            # transitively through `//...`.
+            -//third_party/v8:all
+          )
+
           ./.github/scripts/run-bazel-ci.sh \
             --print-failed-test-logs \
             --use-node-test-env \
@@ -77,8 +84,7 @@ jobs:
             --test_verbose_timeout_warnings \
             --build_metadata=COMMIT_SHA=${GITHUB_SHA} \
             -- \
-            //... \
-            -//third_party/v8:all
+            "${bazel_targets[@]}"
 
       # Save bazel repository cache explicitly; make non-fatal so cache uploading
       # never fails the overall job. Only save when key wasn't hit.