fix(combo): preserve terminal upstream error codes

[shipsfromrio]

Problem

When every target of a combo fails, the aggregated error drops the terminal upstream error code. A Responses client that gets invalid_encrypted_content from the last target sees a generic bad_request instead, so it can't tell that it should drop the replayed reasoning item and retry.

executeTargetAttempt.ts recorded each failure without its structured code, comboAttemptLoop.ts rebuilt the terminal error without a code, and utils/error.ts then fell back to the status-based default. The round-robin path returned a bare { error: { message } } body.

Closes #15139

Fix

• Record structuredError.code on each combo failure entry (priority and round-robin paths).
• resolveComboTerminalCode() keeps the terminal target's code only when it represents the aggregate verdict: same status and the same failure class for every entry. Mixed failures keep today's aggregate status and generic code.
• Round-robin now returns through errorResponse(), so its body gets the same shape and message sanitization as the priority path.

Tests

New tests/unit/combo-error-code-terminal.test.ts: Responses SSE frames, fallback, missing codes, sanitization and mixed failures. Seven assertions fail with the source change reverted. With combo-terminal-status-policy-10501.test.ts, 13/13 pass locally.

[diegosouzapw]

Thanks @shipsfromrio — merged into release/v3.8.52; it ships in the next release.