feat: add GitLab support to coderd/externalauth/gitprovider

Fixes CODAGT-146

Add GitLab support to the gitprovider package for gitsync/chatd PR
diff flows. This is a squashed stack of 3 PRs:

#25651 - refactor(coderd/externalauth): prepare gitprovider for multi-provider support
- Change gitprovider.New to return (Provider, error)
- Extract shared helpers (parseRetryAfter, checkRateLimitError,
  countDiffLines, escapePathPreserveSlashes) from github.go
- Update all callers (db2sdk, exp_chats, gitsync) for new signature
- Add error logging for provider construction failures
- Thread context through provider resolution

#25652 - feat(coderd/externalauth/gitprovider): add GitLab provider
- Implement full Provider interface: FetchPullRequestStatus,
  FetchPullRequestDiff, FetchBranchDiff, ResolveBranchPullRequest
- Handle nested groups, forks, and self-hosted instances
- Rate limit detection on both library and raw HTTP paths
- URL parsing/building with NormalizePullRequestURL support
- Unit tests covering error paths, URL parsing, state mapping
- Document GitLab configuration and known limitations

#25653 - test(coderd/externalauth/gitprovider): add GitLab VCR integration tests
- FetchPullRequestStatus: 4 fixtures (open, conflicts, merged, closed)
- FetchPullRequestDiff: 4 fixtures
- FetchBranchDiff: 3 fixtures (open, deleted, fork)
- ResolveBranchPullRequest: 3 fixtures
- go-vcr cassettes with sanitized GitLab API responses

coderd/externalauth/gitprovider/testdata/gitlab_cassettes/FetchPullRequestDiff/nested_merged.yaml

@@ -0,0 +1,30 @@
+---
+version: 2
+interactions:
+    - id: 0
+      request:
+        proto: HTTP/1.1
+        proto_major: 1
+        proto_minor: 1
+        content_length: 0
+        host: gitlab.com
+        headers:
+            Authorization:
+                - stripped
+            User-Agent:
+                - stripped
+        url: https://gitlab.com/api/v4/projects/test-group9945421%2Ftest-subgroup%2Fanother-test-project/merge_requests/1/raw_diffs
+        method: GET
+      response:
+        proto: HTTP/2.0
+        proto_major: 2
+        proto_minor: 0
+        content_length: -1
+        uncompressed: true
+        body: 'diff --git a/README.md b/README.md\nindex c1dc7b34c381ad6f417bb3f11dba4b1e8f076ff4..b48d45443e349c6dd113da4bb7546504a07a5cce 100644\n--- a/README.md\n+++ b/README.md\n@@ -1,6 +1,6 @@\n # another-test-project\n \n-\n+This is another test project for testing stuff.\n \n ## Getting started\n \n'
+        headers:
+            Content-Type:
+                - text/plain
+        status: 200 OK
+        code: 200
+        duration: 100.000000ms