fix: per-message trace context propagation for persistent sessions

[Syed-Umer-Ali]

Inject the ambient OTel trace context into every user message sent to the CLI (instead of only at connect() time), so outbound MCP/tool spans attribute to the caller's current turn trace in long-lived sessions.

Changes:

• Add _internal/_trace_helpers.py with shared inject_trace_into_message()
• ClaudeSDKClient.query(): inject trace context into string/AsyncIterable messages
• InternalClient._process_query_inner(): inject trace context into initial message
• Query.stream_input(): inject trace context into each streamed message
• Query.set_trace_context(): new control request to update CLI trace context

Closes #1126

[Syed-Umer-Ali]

Reply for knoal:
Thanks for the feedback — valid points. I've added 16 tests covering:

a) Single messages: TestInjectTraceIntoMessage verifies traceparent/tracestate injection, no-op without OTel, no-op without active span, and error handling.

b) Multi-message streams: TestClaudeSDKClientQueryTraceInjection.test_async_iterable_prompt_each_message_gets_trace_context and TestQueryStreamInputTraceInjection.test_stream_input_injects_trace_context verify every message in an AsyncIterable gets its own trace context. TestInternalClientProcessQueryTraceInjection.test_async_iterable_prompt_injects_trace_context covers the query(prompt=stream) path.

c) No propagation without OTel: Both the string and iterable paths have tests verifying traceparent is absent when opentelemetry is not installed.

d) Query.set_trace_context() is tested separately, including no-op and error handling.

All 16 tests pass. PR updated.
Summary of changes pushed to PR #1128:

• tests/test_trace_helpers.py — 16 tests across 5 classes (624 lines)
• Covers: helper unit tests, ClaudeSDKClient.query() (string + stream), query() function (string + stream), Query.stream_input(), Query.set_trace_context()