Migrate GitHub REST tests to internal testify-based mock helper

[Copilot]

Summary

Replaces go-github-mock usage in issues/projects/pullrequests/repositories/search tests with the shared testify-based testmock helper and adjusts raw mock patterns.

Why

Aligns REST test mocking with the new testify pattern used elsewhere.

What changed

• Added pkg/testmock to mirror needed endpoint constants and provide queueable HTTP handlers.
• Swapped affected tests to use testmock imports and endpoint constants; updated raw mock definitions.
• Updated testing docs to reference pkg/testmock instead of go-github-mock.

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

Lint & tests

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

Docs

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

Original prompt

@copilot

migrate following tests from migueleliasweb/go-github-mock to stretchr/testify:
issues_test.go
projects_test.go
pullrequests_test.go
repositories_test.go
search_test.go
raw_mock.go
Follow the pattern in this pr: #1737
Once you are done update the readme and other docs.

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[github-actions]

⚠️ License files need updating

The license files are out of date. I tried to fix them automatically but don't have permission to push to this branch.

Please run:

script/licenses
git add third-party-licenses.*.md third-party/
git commit -m "chore: regenerate license files"
git push

Alternatively, enable "Allow edits by maintainers" in the PR settings so I can fix it automatically.