feat: add a Parallel search option to the deep research example

[georgeatparallel]

Summary

The Deep Research example currently requires a Tavily API key for web search. This adds an explicit SEARCH_PROVIDER=parallel option that uses Parallel's hosted Search MCP endpoint without changing the default Tavily path.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs only
• Tests only
• CI / tooling
• Breaking change

Test plan

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s examples/python_agent_nodes/deep_research/tests -p 'test_*.py' -v
• git diff --check and python3 -m compileall -q examples/python_agent_nodes/deep_research
• Ruff lint and format checks for the changed Python files
• Bounded anonymous request to https://search.parallel.ai/mcp, parsed through the new adapter

Test coverage

This repo enforces a coverage gate on every PR (see
.github/workflows/coverage.yml and
docs/COVERAGE.md).

• The gate runs ./scripts/coverage-summary.sh and compares per-surface
  numbers against coverage-baseline.json.
• It fails if any surface drops more than 1.0 pp below its baseline,
  if any surface is below 80%, or if the weighted aggregate falls below
  85%.

Before asking for review, please confirm:

• I ran tests for the surface(s) I changed locally.
• New code paths are covered by tests in this PR (no bare additions).
• I did not lower or otherwise change coverage-baseline.json.
• The coverage gate check is green in CI before requesting review.

For AI coding agents: if the Coverage Summary GitHub Actions job
fails, read the sticky PR comment titled "📊 Coverage report" — it lists
the specific surface that regressed, the delta, and the exact command to
reproduce locally. Add tests for the uncovered lines in this same PR
before requesting review again. Do not lower baselines to silence the
gate unless the regression is intentional and explicitly called out in
the PR summary.

Checklist

• I have read CONTRIBUTING.md (if present) and
  docs/DEVELOPMENT.md.
• Commits are signed off and follow conventional-commits style.
• I have linked any related issues (none for this focused example change).

Related issues / PRs

None.

The repository-wide CI and coverage checks will run after this draft is opened. The hosted CLA will also require completion through the repository's CLA Assistant check.

Disclosure: I work at Parallel.ai, which operates this MCP endpoint.

[CLAassistant]

All committers have signed the CLA.