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

justfile

@@ -1,6 +1,8 @@
 set working-directory := "codex-rs"
 set positional-arguments
 
+rust_min_stack := "8388608" # 8 MiB
+
 # Display help
 help:
     just -l
@@ -49,7 +51,7 @@ install:
 # Prefer this for routine local runs. Workspace crate features are banned, so
 # there should be no need to add `--all-features`.
 test:
-    cargo nextest run --no-fail-fast
+    RUST_MIN_STACK={{ rust_min_stack }} cargo nextest run --no-fail-fast
 
 # Build and run Codex from source using Bazel.
 # Note we have to use the combination of `[no-cd]` and `--run_under="cd $PWD &&"`