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/core/tests/common/BUILD.bazel

@@ -4,9 +4,6 @@ codex_rust_crate(
     name = "common",
     crate_name = "core_test_support",
     crate_srcs = glob(["*.rs"]),
-    lib_data_extra = [
-        "//codex-rs/core:model_availability_nux_fixtures",
-    ],
     deps_extra = [
         "@crates//:similar",
     ],