feat: implement cursor pagination for dependabot alerts

[MayorFaj]

Summary

Why

Fixes #2649 #2648

What changed

Fixes list_dependabot_alerts pagination by switching it from page-based pagination to cursor-based pagination.

• Removed the unsupported page parameter from list_dependabot_alerts.
• Added cursor pagination support using after and perPage.
• Updated the tool response to include pageInfo, so callers can use pageInfo.nextCursor for the next page.
• Updated tests, toolsnap, and generated docs.

MCP impact

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

Prompts tested (tool changes only)

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

• Updated (README / docs / examples)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates the list_dependabot_alerts tool to use cursor-based pagination and to surface pagination metadata (next cursor) in the tool response.

Changes:

• Switched list_dependabot_alerts from page-based pagination (page) to cursor pagination (after).
• Updated the tool response format to wrap alerts with pageInfo (including nextCursor / hasNextPage).
• Updated tests, tool snapshots, and README to reflect the new pagination input and output shape.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
pkg/github/dependabot.go	Implements cursor pagination for Dependabot alerts and returns alerts plus pageInfo envelope.
pkg/github/dependabot_test.go	Updates tests to use after and validates pageInfo.nextCursor/hasNextPage behavior.
pkg/github/toolsnaps/list_dependabot_alerts.snap	Updates the tool input schema snapshot to remove page and add after.
README.md	Updates documented inputs for list_dependabot_alerts to replace page with after.

[MayorFaj]

This is intentionally changed to match the repo's existing cursor-pagination response pattern. The previous bare-array response could not support Dependabot cursor pagination end-to-end because the next cursor is only available from the response Link header. Returning { alerts, pageInfo } gives callers a usable pageInfo.nextCursor for subsequent requests.