feat: add shared HTTP sessions and configurable timeouts

[Mattsface]

Why

The HTTP adapter used module-level requests.get(), which prevented connection reuse, made timeouts/session injection awkward, and left no clear cleanup path. Every request also needs an explicit timeout so the library cannot hang indefinitely waiting on the MLB API.

What

• Route all adapter HTTP calls through a requests.Session
• Share one session between the Mlb client's v1 and v1.1 adapters
• Add a default timeout of (3.05, 30.0) and allow scalar/tuple overrides
• Support optional session= injection on Mlb / MlbDataAdapter
• Track session ownership: library-created sessions are closed; injected sessions are not
• Add Mlb.close() plus context-manager support (with Mlb() as mlb:)
• Keep existing constructor positional args and adapter response behavior intact
• Add offline coverage in tests/test_mlb_session.py

Tests

• poetry run pytest tests/ --ignore=tests/external_tests — 116 passed
• poetry run pytest tests/external_tests/mlbdataadapter/ — 3 passed

Risk and impact

• Normal — transport plumbing changes, but endpoint signatures, return types, and 404/exception behavior are unchanged
• If something goes wrong: requests could fail on timeout/session mishandling, or callers with custom sessions could see unexpected close behavior (mitigated by ownership tests)

Intentionally left out

• Retries / backoff
• New exception subclasses
• Async client
• Caching / rate limiting
• Endpoint return-type changes