feat(client): add session pooling for connection reuse

[tinosattavily]

Summary

• Add requests.Session() for HTTP connection pooling, reducing TCP/TLS handshake overhead on repeated API calls
• Add close() method and context manager support (with TavilyClient() as client:) for proper resource cleanup
• Add defensive setdefault() for response key normalization in search() and extract()
• Remove deprecated get_company_info() method
• Bump version to 0.7.20

Test plan

• Unit tests pass (pytest tests/ - 21 passed)
• Local manual tests pass (search, extract, crawl, map, context manager, session reuse)
• QA smoke tests pass (13 passed)
• QA regression tests pass (98 passed, 10 failed due to API behavior mismatches unrelated to this change)

Closes TAV-4105

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