Remove CODEX_RS_SSE_FIXTURE test hook (#22413)

## Why

`CODEX_RS_SSE_FIXTURE` let integration-style CLI, exec, and TUI tests
bypass the normal Responses transport by reading SSE from local files.
That kept test-only behavior wired through production client code. The
affected tests can stay hermetic by using the existing
`core_test_support::responses` mock server and passing `openai_base_url`
instead.

## What Changed

- Removed the `CODEX_RS_SSE_FIXTURE` flag,
`codex_api::stream_from_fixture`, the `env-flags` dependency, and the
checked-in SSE fixture files.
- Repointed the affected core, exec, and TUI tests at `MockServer` with
the existing SSE event constructors.
- Removed the Bazel test data plumbing for the deleted fixtures and
refreshed cargo/Bazel lock state.

## Verification

- `cargo build -p codex-cli`
- `cargo test -p codex-api`
- `cargo test -p codex-core --test all responses_api_stream_cli`
- `cargo test -p codex-core --test all
integration_creates_and_checks_session_file`
- `cargo test -p codex-exec --test all ephemeral`
- `cargo test -p codex-exec --test all resume`
- `cargo test -p codex-tui --test all
resume_startup_does_not_consume_model_availability_nux_count`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just fix -p codex-api -p codex-core -p codex-exec -p codex-tui`
- `git diff --check`

codex-rs/Cargo.lock

@@ -2536,7 +2536,6 @@ dependencies = [
  "ctor 0.6.3",
  "dirs",
  "dunce",
- "env-flags",
  "eventsource-stream",
  "futures",
  "http 1.4.0",
@@ -3791,6 +3790,7 @@ dependencies = [
  "codex-utils-string",
  "codex-windows-sandbox",
  "color-eyre",
+ "core_test_support",
  "cpal",
  "crossterm",
  "derive_more 2.1.1",
@@ -3841,6 +3841,7 @@ dependencies = [
  "which 8.0.0",
  "windows-sys 0.52.0",
  "winsplit",
+ "wiremock",
 ]
 
 [[package]]
@@ -5389,12 +5390,6 @@ dependencies = [
  "syn 2.0.114",
 ]
 
-[[package]]
-name = "env-flags"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "dbfd0e7fc632dec5e6c9396a27bc9f9975b4e039720e1fd3e34021d3ce28c415"
-
 [[package]]
 name = "env_filter"
 version = "1.0.0"