fix(stream): report upstream interruption instead of faking success

[lzhs1995]

Problem

When the upstream stream breaks mid-response, both SSE paths called the normal
completion routine:

• streaming (create_sse_stream, Some(Err(e))) -> ctx.generate_final_events()
• buffered (create_buffered_sse_stream, Some(Err(e))) -> ctx.finish_and_get_all_events()

Both emit message_delta with stop_reason: end_turn plus message_stop. The trace
was already recorded as interrupted, but the client saw a fully successful turn.

Consequence for Claude Code CLI: a truncated response is treated as a finished turn,
so the client does not retry. Half-written tool arguments are silently dropped and
multi-step tasks "complete" in the middle.

Fix

New interruption path that closes any open content blocks and emits an Anthropic
error event with type: "overloaded_error" (a retryable type), and never
emits message_delta / message_stop:

• SseStateManager::generate_interrupted_events() - closes open blocks, sets the
  message_delta_sent / message_ended idempotency flags so no later path can
  append a normal completion.
• StreamContext::generate_interrupted_events(message) - the above plus the error
  event. Deliberately performs no flush of thinking / invoke-sniff / XML-filter
  residue and no tool-JSON accumulator finish: that residue is incomplete data, and
  flushing it would dress up half content as valid content.
• BufferedStreamContext::interrupt_and_get_all_events(message) - emits the buffered
  events (useful for diagnosis) with the same error tail and the same usage
  correction on message_start as the success path.

Err only ever comes from a transport failure; normal completion is None (EOF),
which still goes through the unchanged success path.

Tests

6 new tests in src/anthropic/stream.rs:

test	locks
interrupted_stream_emits_error_and_no_normal_completion	exactly one overloaded_error, it is last, open blocks closed, no message_delta/message_stop
interrupted_stream_cannot_regress_to_fake_success	a later generate_final_events() cannot append a normal completion
interrupted_stream_does_not_flush_truncated_tool_json	half-written tool args never surface as a tool_use block
buffered_interrupted_stream_emits_error_and_no_normal_completion	same contract in buffered mode, buffered events retained
buffered_interrupted_stream_cannot_regress_to_fake_success	same anti-regression for buffered mode
normal_completion_still_emits_message_stop	reverse control: success path unchanged (end_turn + message_stop, no error)

Full suite: cargo test --locked --all-targets --no-default-features -> 577 passed, 0 failed.
rustfmt --edition 2024 --check clean on the CI gate file list.

CI

• Candidate workflow trigger extended to this branch.
• src/anthropic/stream.rs added to the rustfmt gate (it is now a candidate file).
• Clippy stays diagnostic-only; the 179 findings on this tree are the pre-existing
  upstream baseline and none are in the two files touched here.

Attribution

Behaviour reference: the overloaded_error-on-interruption approach in
dmcyfelix/kiro2cc-proxy@50bd8d0. No code copied.

Not in scope

stopReason / metadataEvent parsing (Quorinex/Kiro-Go#141) was dropped: raw
EventStream captures against our own upstream endpoint show zero stopReason
fields and no metadataEvent frames, so there is nothing to parse.

[Copilot]

Pull request overview

This PR changes the Anthropic SSE streaming implementation so that an upstream transport interruption is surfaced to clients as an explicit Anthropic error event (retryable overloaded_error) instead of emitting a normal successful completion (message_delta with stop_reason: end_turn + message_stop). This prevents clients (notably Claude Code CLI) from treating truncated responses as completed turns and silently continuing without retrying.

Changes:

• Add an explicit “interrupted” termination path that closes open content blocks, marks completion idempotency flags, and emits a terminal error event (no message_delta / message_stop).
• Wire the new interruption path into both streaming and buffered SSE handlers for Some(Err(_)) upstream read failures.
• Extend the document-candidate workflow trigger and rustfmt gate list to include the touched stream file.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/anthropic/stream.rs	Introduces interrupted termination event generation and adds regression tests ensuring interruptions don’t fake success.
src/anthropic/handlers.rs	Routes upstream read errors to the new interrupted termination path in both streaming and buffered modes.
.github/workflows/kiro-document-candidate.yml	Updates branch trigger list and adds stream.rs to the rustfmt gate file list.

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