fix: refactor existing PR message format when calling create_pull_req…

[Nishthajain7]

Summary

Catch the "pull request already exists" 422 from create_pull_request and return a clean message with the existing PR's URL instead of forwarding GitHub's raw, unformatted validation error.

Why

When create_pull_request fails because a PR already exists for the given head/base, the tool currently forwards GitHub's raw API error verbatim (e.g. failed to create pull request: POST .../pulls: 422 Validation Failed [{Resource:PullRequest Field: Code:custom Message:A pull request already exists for owner:branch.}]). This is unreadable and gives the caller no way to find the existing PR without a separate lookup.

Fixes #2931

What changed

• Added findExistingPullRequest, which detects the specific code:"custom" / "A pull request already exists" 422 shape and calls PullRequests.List (filtered by head/base) to resolve the existing PR
• Updated CreatePullRequest's error handling: on this specific error, return a structured {id, url, message} result pointing at the existing PR instead of the raw error; all other Create errors are unaffected and still return the original error path

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

"Create a pull request from test/dup-pr into main" (when a PR already exists for that branch pair) returns the existing PR's number and link instead of an error

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

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