Add the utility to truncate by tokens (#6746)

- This PR is to make it on path for truncating by tokens. This path will
be initially used by unified exec and context manager (responsible for
MCP calls mainly).
- We are exposing new config `calls_output_max_tokens`
- Use `tokens` as the main budget unit but truncate based on the model
family by Introducing `TruncationPolicy`.
- Introduce `truncate_text` as a router for truncation based on the
mode.

In next PRs:
- remove truncate_with_line_bytes_budget
- Add the ability to the model to override the token budget.

codex-rs/core/src/context_manager/mod.rs

@@ -1,7 +1,5 @@
 mod history;
 mod normalize;
 
-pub(crate) use crate::truncate::MODEL_FORMAT_MAX_BYTES;
-pub(crate) use crate::truncate::MODEL_FORMAT_MAX_LINES;
-pub(crate) use crate::truncate::format_output_for_model_body;
+pub(crate) use crate::truncate::truncate_with_line_bytes_budget;
 pub(crate) use history::ContextManager;