Implement AsyncMlb vertical slice

[Mattsface]

Parent: #297
Depends on: #299, #301, #302

Goal

Add the first public AsyncMlb client with a deliberately small set of representative endpoints to prove the end-to-end architecture before expanding coverage.

Initial scope

Implement AsyncMlb with:

• public root import: from mlbstatsapi import AsyncMlb
• constructor shape aligned with Define the v1.1 async API and transport contract #298
• async with AsyncMlb() support
• explicit aclose()
• shared async transport lifecycle
• existing strict/compatibility settings from the async contract
• initial endpoints:
  • get_team
  • get_person
  • get_schedule

Contract requirements from #298

Public API

• AsyncMlb is a stable public 1.x API
• Public async endpoint names/signatures become compatibility commitments once released
• Existing synchronous Mlb behavior and signatures remain unchanged
• Return the same existing Pydantic/domain models as sync equivalents
• Preserve the same None, [], and {} behavior for equivalent responses

Constructor and lifecycle

Public shape should mirror sync as closely as practical:

AsyncMlb(
    hostname="statsapi.mlb.com",
    logger=None,
    timeout=DEFAULT_TIMEOUT,
    client=None,
    *,
    strict_http=True,
)

• async with returns the AsyncMlb instance
• __aexit__() awaits aclose()
• aclose() is idempotent and closes only library-owned async transport
• Caller-injected clients remain caller-owned and open
• Cleanup must preserve original exceptions and cancellation

Endpoint behavior

For get_team, get_person, and get_schedule:

• Equivalent response fixtures should produce the same domain results as sync
• Endpoint-specific 404 behavior must match sync
• Strict/compatibility HTTP semantics and public exceptions come from Define the v1.1 async API and transport contract #298/Implement AsyncMlbDataAdapter #301
• Shared parsing extracted in Extract reusable endpoint parsing for sync/async clients #299 should be reused rather than duplicated

Concurrency

• One AsyncMlb instance supports multiple concurrent in-flight endpoint calls on the same event loop
• Concurrency remains caller-controlled
• Endpoint methods do not introduce hidden fan-out, detached tasks, worker pools, or unrelated background work
• Per-request state remains independent
• Caller-created in-flight operations must finish before the client is closed

Design requirement

Do not duplicate parsing/model-construction logic when a shared helper from #299 can be used.

Transport-specific implementation details must remain behind AsyncMlbDataAdapter rather than leaking into the public AsyncMlb API.

Acceptance criteria

• AsyncMlb is importable from mlbstatsapi
• Constructor/lifecycle behavior matches Define the v1.1 async API and transport contract #298
• async with correctly cleans up library-owned resources
• Explicit aclose() is idempotent and safe
• get_team, get_person, and get_schedule work end to end
• Returned models and empty-result behavior match sync equivalents
• Initial endpoints reuse shared parsing/model construction
• Same-client concurrent endpoint calls work on one event loop
• Endpoint calls do not introduce hidden fan-out/background work
• Existing Mlb behavior and signatures remain unchanged
• Initial public async usage has deterministic tests

Refs #297
Contract: #298