hooks: emit Bash PostToolUse when exec_command completes via write_stdin (#18888)

Fixes #16246.

## Why

`exec_command` already emits `PreToolUse`, but long-running unified exec
commands that finish on a later `write_stdin` poll could miss the
matching `PostToolUse`. That left the Bash hook lifecycle inconsistent,
broke expectations around `tool_use_id` and `tool_input.command`, and
meant `PostToolUse` block/replacement feedback could fail to replace the
final session output before it reached model context.

This keeps the fix scoped to the `exec_command` / `write_stdin`
lifecycle. Broader non-Bash hook expansion is still out of scope here
and remains tracked separately in #16732.

## What changed

- Compute and store `PostToolUsePayload` while handlers still have
access to their concrete output type, and carry `tool_use_id` through
that payload.
- Preserve the original hook-facing `exec_command` string through
unified exec state (`ExecCommandRequest`, `ProcessEntry`,
`PreparedProcessHandles`, and `ExecCommandToolOutput`) via
`hook_command`, and remove the now-unused `session_command` output
metadata.
- Emit exactly one Bash `PostToolUse` for long-running `exec_command`
sessions when a later `write_stdin` poll observes final completion,
using the original `exec_command` call id and hook-facing command.
- Keep one-shot `exec_command` behavior aligned with the same payload
construction, including interactive completions that return a final
result directly.
- Apply `PostToolUse` block/replacement feedback before the final
`write_stdin` completion output is sent back to the model.
- Keep `write_stdin` itself out of `PreToolUse` matching so it continues
to act as transport/polling for the original Bash tool call.
- Restore plain matcher behavior for tool-name matchers such as `Bash`
and `Edit|Write`, while still treating patterns with regex characters
(for example `mcp__.*`) as regexes.
- Add unit coverage for unified exec payload construction and parallel
session separation, plus a core integration regression that verifies a
blocked `PostToolUse` replaces the final `write_stdin` output in model
context.

## Testing

- `cargo test -p codex-hooks`
- `cargo test -p codex-core post_tool_use_payload`
- `cargo test -p codex-core
post_tool_use_blocks_when_exec_session_completes_via_write_stdin`

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

@@ -148,7 +148,7 @@ struct ProcessEntry {
     process: Arc<UnifiedExecProcess>,
     call_id: String,
     process_id: i32,
-    command: Vec<String>,
+    hook_command: String,
     tty: bool,
     network_approval_id: Option<String>,
     session: Weak<Session>,