fix: serialize VCS tests to prevent race conditions (EME-368) (#2811)

## Summary
- Fixes flaky `test_find_base_sha` test that was consistently failing on
Windows CI
- Adds `serial_test` dependency to serialize tests that share the
`GITHUB_EVENT_PATH` environment variable
- Adds `#[serial(github_event_path)]` attribute to both conflicting
tests

## Root Cause
The flakiness was caused by a race condition where two tests run in
parallel and both manipulate the same global `GITHUB_EVENT_PATH`
environment variable:

- `test_find_base_sha()` 
- `test_find_head_with_github_event_path()`

When running in parallel (Rust's default), these tests could overwrite
each other's environment variable settings, causing random failures.

## Solution  
Using the `serial_test` crate with `#[serial(github_event_path)]`
ensures these specific tests run sequentially while allowing other tests
to continue running in parallel.


Closes EME-368

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Serializes two VCS tests using serial_test to avoid GITHUB_EVENT_PATH
race; adds serial_test as a dev dependency.
> 
> - **Tests**:
> - Add `#[serial(github_event_path)]` to
`test_find_head_with_github_event_path` and `test_find_base_sha` in
`src/utils/vcs.rs`.
> - **Dependencies**:
>   - Add dev-dependency `serial_test` in `Cargo.toml`.
> - Update `Cargo.lock` with `serial_test` and transitive deps (`scc`,
`sdd`) and include `serial_test` in `sentry-cli` package deps.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c6c76f3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: runningcode

Cargo.lock

@@ -86,6 +86,7 @@ assert_cmd = "2.0.11"
 insta = { version = "1.26.0", features = ["redactions", "yaml"] }
 mockito = "1.6.1"
 rstest = "0.18.2"
+serial_test = "3.1.1"
 tempfile = "3.8.1"
 trycmd = "0.14.11"
 

Cargo.toml

@@ -806,6 +806,7 @@ mod tests {
     use {
         crate::api::RepoProvider,
         insta::{assert_debug_snapshot, assert_yaml_snapshot},
+        serial_test::serial,
         std::fs::File,
         std::io::Write as _,
         std::path::Path,
@@ -1672,6 +1673,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(github_event_path)]
     fn test_find_head_with_github_event_path() {
         use std::fs;
 
@@ -1751,6 +1753,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(github_event_path)]
     fn test_find_base_sha() {
         use std::fs;