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

Scope boundary with #302

#302 locks down the deterministic contract for AsyncMlbDataAdapter and the transport layer. This issue owns the higher-level AsyncMlb lifecycle that cannot be tested until AsyncMlb exists.

In particular, #303 owns deterministic coverage for:

• async with AsyncMlb(...)
• AsyncMlb.__aenter__() returning the AsyncMlb instance
• AsyncMlb.__aexit__() awaiting cleanup
• automatic cleanup on normal context exit
• automatic cleanup when an exception leaves the async context
• automatic cleanup when the surrounding task is cancelled
• preserving the original exception or asyncio.CancelledError while cleanup runs
• keeping caller-injected clients open during all of the above

Adapter-level ownership, explicit AsyncMlbDataAdapter.aclose(), retry, transport, and cancellation behavior remain covered by #301/#302 and should not be duplicated here except where needed to prove the public AsyncMlb behavior end to end.

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
• Context-manager cleanup behavior on normal exit, exception, and cancellation is part of this issue's deterministic test coverage

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.

Do not duplicate the exhaustive adapter-level contract matrix from #302. Add only the transport assertions required to prove the public AsyncMlb lifecycle and endpoint behavior end to end.

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 on normal exit
• async with correctly cleans up library-owned resources when an exception leaves the context
• async with correctly cleans up library-owned resources when the surrounding task is cancelled
• Context-manager cleanup preserves the original exception or asyncio.CancelledError
• Caller-injected clients remain open across explicit and context-manager cleanup
• 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 without duplicating Add deterministic async transport contract tests #302's adapter-level contract matrix

Refs #297
Contract: #298

[Mattsface]

Implementation note for get_schedule:

AsyncMlb.get_schedule() should mirror the existing synchronous Mlb.get_schedule() public signature exactly rather than introducing a simplified **params-only shape. The async API should differ only where async semantics require it (await / async lifecycle), not in endpoint argument design.

Before implementing or merging the method, compare its full signature and defaults directly against Mlb.get_schedule() and add deterministic coverage that protects the intended signature parity.