test(coding-agent): add issue #900 context eligibility regression

[Mustaqeem66]

Failed assistant retries and compaction_outcome diagnostics must remain durable in session entries while being excluded from both the provider context and the compaction input.

Note

Add regression test for issue #900 context eligibility in compaction recovery

Adds a vitest regression suite in 900-compaction-recovery.test.ts to verify that failed assistant messages and compaction outcome messages are excluded from model context and compaction input while remaining in durable history.

• Introduces a failedAssistant helper that builds a consistent AssistantMessage with stopReason: 'error' and errorMessage: 'context_length_exceeded'.
• The test builds a conversation with an old user message, a failed assistant message, a compaction outcome message, and a recent user message, then asserts convertToLlm returns only the two user messages.
• Verifies estimateRawContextTokens excludes the failed/outcome messages and that prepareCompaction schedules only the old user message for summarization with an empty turnPrefixMessages.

^{Macroscope summarized 3515afb.}