test: set Rust test thread stack size (#19067)

## Summary

Set `RUST_MIN_STACK=8388608` for Rust test entry points so
libtest-spawned test threads get an 8 MiB stack.

The Windows BuildBuddy failure on #18893 showed
`//codex-rs/tui:tui-unit-tests` exiting with a stack overflow in a
`#[tokio::test]` even though later test binaries in the shard printed
successful summaries. Default `#[tokio::test]` uses a current-thread
Tokio runtime, which means the async test body is driven on libtest's
std-spawned test thread. Increasing the test thread stack addresses that
failure mode directly.

To date, we have been fixing these stack-pressure problems with
localized future-size reductions, such as #13429, and by adding
`Box::pin()` in specific async wrapper chains. This gives us a baseline
test-runner stack size instead of continuing to patch individual tests
only after CI finds another large async future.

## What changed

- Added `common --test_env=RUST_MIN_STACK=8388608` in `.bazelrc` so
Bazel test actions receive the env var through Bazel's cache-keyed test
environment path.
- Set the same `RUST_MIN_STACK` value for Cargo/nextest CI entry points
and `just test`.
- Annotated the existing Windows Bazel linker stack reserve as 8 MiB so
it stays aligned with the libtest thread stack size.

## Testing

- `just --list`
- parsed `.github/workflows/rust-ci.yml` and
`.github/workflows/rust-ci-full.yml` with Ruby's YAML loader
- compared `bazel aquery` `TestRunner` action keys before/after explicit
`--test_env=RUST_MIN_STACK=...` and after moving the Bazel env to
`.bazelrc`
- `bazel test //codex-rs/tui:tui-unit-tests --test_output=errors`
- failed locally on the existing sandbox-specific status snapshot
permission mismatch, but loaded the Starlark changes and ran the TUI
test shards

.github/workflows/rust-ci-full.yml

@@ -76,6 +76,8 @@ jobs:
       - name: Test argument comment lint package
         working-directory: tools/argument-comment-lint
         run: cargo test
+        env:
+          RUST_MIN_STACK: "8388608" # 8 MiB
 
   argument_comment_lint_prebuilt:
     name: Argument comment lint - ${{ matrix.name }}
@@ -671,6 +673,7 @@ jobs:
         run: cargo nextest run --no-fail-fast --target ${{ matrix.target }} --cargo-profile ci-test --timings
         env:
           RUST_BACKTRACE: 1
+          RUST_MIN_STACK: "8388608" # 8 MiB
           NEXTEST_STATUS_LEVEL: leak
 
       - name: Upload Cargo timings (nextest)