feat: Add ChatCompletionsHTTPClient and support for non-streaming requests

[copybara-service]

feat: Add ChatCompletionsHTTPClient and support for non-streaming requests

This is part of a larger chain of commits for adding chat completion API support to the Apigee model.

The HTTP client wraps payload construction (delegating to ChatCompletionsRequest.fromLlmRequest) and response parsing (delegating to ChatCompletionsResponse.ChatCompletion / ChatCompletionChunkCollection) for both non-streaming and streaming Server-Sent Events responses.
END_PUBLIC

Key behaviors:

• Tri-state call timeout policy:

  • httpOptions == null OR timeout() empty: applies a default 5-minute call
    timeout to prevent indefinite hangs in the common unconfigured case.
  • httpOptions.timeout() == 0: respected as the explicit caller opt-in to
    infinite hang for long-running streams or batch jobs.
  • httpOptions.timeout() > 0: applied directly as the call timeout.
    This default intentionally diverges from the GenAI HttpOptions convention
    (which treats unset as infinite) as a defensive measure since this client
    does not yet have HTTP retry support.

• SSE prefix handling accepts both "data: foo" (with space) and "data:foo"
  (without space) per the SSE spec, matching providers that omit the
  trailing space.

• A single malformed JSON chunk in a streaming response is logged and
  skipped rather than aborting the entire stream. IOException
  (connection-level) still propagates as a stream error.

• Content-Type is defensively forced to application/json by replacing
  rather than appending, preventing duplicate or conflicting headers if a
  caller supplies their own Content-Type.

• Headers parameter accepts null (treated as no extra headers) and is
  stored as an ImmutableMap for thread-safe reuse across concurrent
  generateContent calls.

Test additions (16 total, +12 new):

• HTTP error status (4xx/5xx) propagation for both streaming and
  non-streaming.
• Empty body propagation.
• Streaming continues past a single malformed chunk.
• SSE "data:" prefix accepted with or without trailing space.
• Custom headers reach the wire.
• Caller-supplied Content-Type is overridden, not appended.
• baseUrl with and without trailing slash.
• Constructor tri-state timeout (null, zero=infinite, positive).
• Constructor null headers parameter.
  All testSubscriber.await() calls bounded to 500ms to prevent test hangs.