[copybara-service]

feat!: Switch default HTTP client to httpx2 and use OS trust store by default

BREAKING CHANGE: httpx2 is now the required dependency and default HTTP client
instead of httpx, and default SSL verification with httpx2 uses the OS
certificate trust store (via truststore / httpx2.create_ssl_context()) when
SSL_CERT_FILE and SSL_CERT_DIR are not set. To maintain baseline behavior,
install google-genai[httpx] and set GOOGLE_GENAI_HTTP_CLIENT=httpx.

Summary of impact:

• If your code uses httpx or httpx2 to interact with the SDK, make sure it
  uses the same package as the active SDK client.
• In most cases, updating your code with import httpx2 as httpx is sufficient.
• The SDK still fully supports both httpx and httpx2; only the default has
  changed.
• httpx is no longer installed by default (use pip install google-genai[httpx]
  to install a compatible version; google-genai[httpx2] remains as a no-op
  extra).
• You can set the SDK's default backend with GOOGLE_GENAI_HTTP_CLIENT (httpx
  or httpx2 strictly requires that package; unset or auto prefers httpx2
  and falls back to httpx if only httpx is installed).
• Regardless of the default, passing HttpOptions(httpx_client=..., httpx_async_client=...)
  uses those objects directly (sync and async are configured independently).

Where httpx/httpx2 types must match the active client:

• HttpOptions(client_args=..., async_client_args=...): package-specific objects
  such as Timeout, MockTransport / BaseTransport (and Response objects
  from mock handlers), and custom Auth subclasses must match the active client.
  Primitives (numeric timeouts, auth tuples, proxy URL strings) work with either.
• Transport exceptions on client.models, files, etc.: network and timeout
  errors (TimeoutException, ConnectError, HTTPError) are reraised unwrapped
  after retries, so except clauses catching them directly must match the active
  client. (4xx/5xx APIError exceptions and client.interactions
APIConnectionError / APITimeoutError are unchanged, though .response /
  .request attributes come from the active client.)

Fixes: #2680