refactor(pullrequests): convert PR tools to NewTool pattern

Convert all 10 pull request tool functions to use the NewTool
pattern with ToolDependencies injection:
- PullRequestRead
- CreatePullRequest
- UpdatePullRequest
- ListPullRequests
- MergePullRequest
- SearchPullRequests
- UpdatePullRequestBranch
- PullRequestReviewWrite
- AddCommentToPendingReview
- RequestCopilotReview

Update tools.go to use direct function calls (removing
NewServerToolLegacy wrappers) for PR functions.

Update all tests in pullrequests_test.go to use the new
handler pattern with deps and 2-value return.

Co-authored-by: Adam Holt <omgitsads@users.noreply.github.com>

Author: SamMorrowDrums