Add support for updating pull request reviews without dismissal

Describe the feature or problem you’d like to solve

Currently, when bots or agents post reviews on pull requests and new commits arrive, they must dismiss the old review and create a new one.
This clutters the PR timeline with “dismissed” markers, making it harder for developers to follow the real state of reviews.

Proposed solution

Introduce a new MCP tool update_pull_request_review that allows updating an existing pull request review in place.
This enables bots/agents to refresh their review (body, event, commit SHA) without dismissing and recreating, keeping timelines clean.

Benefits:

Cleaner PR timelines (no extra “dismissed” markers).

Allows agents/bots to maintain their review state across new commits.

Complements the existing dismiss_pull_request_review tool for compliance/invalidation cases.

Example workflows (for tools/toolsets only)

Agent detects a new commit → updates its previous review with refreshed comments instead of dismissing/reposting.

Bot changes review from APPROVE to REQUEST_CHANGES after re-analysis of updated code.

Service keeps its review tied to the correct commit SHA without posting duplicate reviews.

Additional context

This was implemented along with:

Full unit test coverage (success, failure, validation cases).

Snapshot + README.md documentation generated.

Because MCP servers are stateless, it’s the agent’s responsibility to track commit IDs and decide when to refresh reviews.

This tool is especially useful for agentic workflows, where automated review agents need to keep feedback synchronized without cluttering the PR history.

**Related PR:** https://github.com/github/github-mcp-server/pull/1149