Improve dependabot error message

[aashah]

This pull request improves error handling for Dependabot alert API failures by providing more informative error messages when access is denied due to insufficient token permissions. It also updates and extends tests to verify these enhanced messages.

Why

When in environments where the github token is provided, like a codespace, the error message is a bit vague in helping the user understand why their tool call failed.

MCP impact

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

Prompts tested (tool changes only)

n/a

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)

[Copilot]

Pull request overview

This PR improves the user-facing error message for Dependabot alert API failures by adding a permission/scope hint when requests are denied, and updates unit tests to validate the new messaging.

Changes:

• Enhance Dependabot tool error messages by appending a token-permission hint for certain HTTP failures.
• Update/extend Dependabot tool tests to assert the improved error output for forbidden/not-found scenarios.

Show a summary per file

File	Description
pkg/github/dependabot.go	Adds dependabotErrMsg helper to enrich Dependabot API error messages with token permission guidance.
pkg/github/dependabot_test.go	Updates and extends tests to assert the new token-permission hint is present in error results.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2