feat(tools): add StackOverflow MCP tool

[yaman-694]

Add a public StackOverflow integration backed by the StackExchange API, including search, question lookup, answer lookup, docs, and tests.

Refs #2805

Description

Add a public StackOverflow integration backed by
the StackExchange API so Hive agents can search
programming questions, fetch full question
content, and retrieve answers with code examples.

Type of Change

• Bug fix (non-breaking change that fixes an
  issue)
• New feature (non-breaking change that adds
  functionality)
• Breaking change (fix or feature that would
  cause existing functionality to not work as
  expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Refs #2805

Changes Made

• Added a new unverified stackoverflow_tool
  integration backed by the public StackExchange
  API
• Implemented stackoverflow_search,
  stackoverflow_get_question, and
  stackoverflow_get_question_answers
• Normalized API responses to include body text,
  extracted code snippets, and rate-limit metadata
• Registered the integration in the central
  tools registry
• Added README documentation for setup, usage,
  and API behavior
• Added unit tests covering search, question
  lookup, answer lookup, validation, and error
  handling

Testing

Describe the tests you ran to verify your
changes:

• Unit tests pass (cd tools && uv run pytest tests/tools/test_stackoverflow_tool.py -q)
• Tools test suite passes (cd tools && uv run pytest tests/ -q)
• Lint passes (make check)
• Manual testing performed

Checklist

• My code follows the project's style
  guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in
  hard-to-understand areas
• I have made corresponding changes to the
  documentation
• My changes generate no new warnings
• I have added tests that prove my fix is
  effective or that my feature works
• New and existing unit tests pass locally
  with my changes

Screenshots (if applicable)

NA

[github-actions]

PR Requirements Warning

This PR does not meet the contribution requirements.
If the issue is not fixed within ~24 hours, it may be automatically closed.

PR Author: @yaman-694
Found issues: #2805 (assignees: none)
Problem: The PR author must be assigned to the linked issue.

To fix:

1. Assign yourself (@yaman-694) to one of the linked issues
2. Re-open this PR

Exception: To bypass this requirement, you can:

• Add the micro-fix label or include micro-fix in your PR title for trivial fixes
• Add the documentation label or include doc/docs in your PR title for documentation changes

Micro-fix requirements (must meet ALL):

Qualifies	Disqualifies
< 20 lines changed	Any functional bug fix
Typos & Documentation & Linting	Refactoring for "clean code"
No logic/API/DB changes	New features (even tiny ones)

Why is this required? See #472 for details.