fix: preserve structured exhaustion on provider embedding stack

[seonghobae]

Summary

• stack the structured-synthesis provider-failure fix on PR fix: use provider-backed embeddings for remote agents #857 provider-backed embeddings
• preserve the stack's existing 413 cross-provider handling
• map virtual structured synthesis exhaustion to the existing redacted 503 no_viable_agent retry contract

Runtime diagnosis

The canonical LineageWeave runtime at orchestrator pin 17a7b13f5317e930bae496e6d65268fb15ac4e34 reproduced synthetic orchestrator/auto + response_format=json_object calls returning 500 internal_error before readiness quarantine changed later requests to typed unavailability. The failure is in final provider-facing synthesis, not embeddings or the Lineage parser. No real source identifiers or content were inspected or recorded.

Validation

• exact fix: use provider-backed embeddings for remote agents #857 base: 63a6e65e8008d275bb765b4e948d505dc9e3cc13 (contains deployed 17a7b13f)
• make test built the pinned Rust token packer and pinned Rust-backed fast-mlsirm successfully
• full stack run: 2748 passed, 2 skipped, 1 failed only because an existing assertion expected the raw provider RuntimeError now intentionally redacted
• after updating that expectation, Docker-native affected rerun: 70 passed
• focused provider-backed embedding + structured path run: 30 passed
• git diff --check: passed

This PR is intentionally stacked on #857 and must not merge independently before its base.

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[devin-ai-integration]

Devin Review found 2 potential issues.