Preserve HTTPX environment proxy behavior with async retry transport

[Mattsface]

Problem

PR #323 moved async retries into a custom HTTPX transport:

httpx.AsyncClient(
    headers={"User-Agent": _build_user_agent()},
    transport=MlbAsyncRetryTransport(),
)

HTTPX only auto-loads environment proxies when transport is None. Passing a custom transport can therefore bypass HTTP_PROXY, HTTPS_PROXY, ALL_PROXY, and NO_PROXY for library-created async clients.

Before #323, the library-created client was effectively:

httpx.AsyncClient(
    headers={"User-Agent": _build_user_agent()},
)

so normal environment proxy handling remained available.

Example

export HTTPS_PROXY=http://proxy.example.com:8080

from mlbstatsapi import AsyncMlb

async with AsyncMlb() as mlb:
    team = await mlb.get_team(133)

Expected: the request follows HTTPX's normal environment proxy behavior.

Acceptance criteria

• Preserve retry transport behavior from refactor(async): move retries and client ownership onto a shared transport #323.
• Preserve normal HTTPX environment proxy handling for library-created clients.
• Keep caller-injected clients untouched.
• Add deterministic tests covering proxy/environment behavior.
• Do not change the public AsyncMlb or AsyncMlbDataAdapter constructor signatures.