Overhaul provider streaming on a shared SSE base

[iskakaushik]

The OpenAI and Anthropic stream implementations had drifted into
near-identical copies of the same threading, queueing, and SSE
parsing code, and both mishandled several streaming cases: tool-call
deltas were never assembled into events, finish events carried no
usage or finish reason, API error bodies on non-200 responses were
lost, and a stream could emit several terminal events.

Introduce providers::streaming::HttpSseStream, which owns the stream
thread, event queue, HTTP request and SSE line splitting, pending
tool-call assembly, and emit-once finish semantics. Each provider
implementation now only translates its SSE lines into stream events.

Behavioral fixes that come with the rework:

• Streamed tool calls are accumulated per index and emitted as
  tool-call events, including when a gateway closes the stream
  without [DONE] or content_block_stop.
• Exactly one finish event terminates every stream, carrying usage
  and the real finish reason on success and kFinishReasonError with
  no usage on failure. OpenAI streams now request
  stream_options.include_usage, without which the API omits usage
  from streaming responses entirely.
• Error bodies of non-200 responses are captured via a response
  handler (httplib routes them through the content receiver, leaving
  response.body empty) so auth and rate-limit failures stay
  diagnosable. A user-initiated stop is no longer reported as a
  network error.
• Anthropic SSE parsing tolerates present-but-null JSON fields from
  gateways, keeps a tool_use block's initial input separate from
  streamed input_json_delta fragments, completes on message_stop
  instead of waiting for the connection to close, and no longer
  races or terminates on a second start_stream call.

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