feat(async-client): add session pooling for connection reuse TAV-4105

[tinosattavily]

Summary

• Add persistent httpx.AsyncClient instance for connection pooling in AsyncTavilyClient
• Add close() method and async context manager support (__aenter__, __aexit__)
• Enable TCP connection reuse across multiple API requests for improved performance
• Add comprehensive test suite for session pooling edge cases (tests/test_session_pooling.py)
• Bump version to 0.7.21

Changes

The async client now maintains a single httpx.AsyncClient instance instead of creating a new client per request. This enables HTTP connection reuse (avoiding TCP/TLS handshake overhead on subsequent requests).

Usage

# Option 1: Context manager (recommended)
async with AsyncTavilyClient(api_key="...") as client:
    result = await client.search("query")
    # Connection is reused for additional requests
    result2 = await client.search("another query")

# Option 2: Manual cleanup
client = AsyncTavilyClient(api_key="...")
try:
    result = await client.search("query")
finally:
    await client.close()

Test plan

• All existing unit tests pass (30 tests)
• New session pooling tests pass (tests/test_session_pooling.py)
• Manual testing with real API calls

Linear: TAV-4105

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