fix(server): surface actionable provider failure messages

[juliusmarminge]

Provider startup and stream failures can collapse actionable errors into "Provider turn failed." For example, a context handoff budget rejection loses the instruction to compact the conversation or choose a model with more context.

Translate known failure categories through wrapped adapter and Effect causes in the shared formatter. Handoff budget and uncertain-delivery errors retain recovery guidance; startup, stream, session-open, and resume failures get specific next steps. Unknown defect text stays internal. Preserve explicit messages, credential redaction, and message length limits, with guarded and bounded cause traversal.

Targets #2829. This changes error reporting only; it adds no legacy reconciliation or database migration.

Verification:

• 50 focused tests passed, including wrapped handoff errors, persisted startup/stream failures, and unreadable causes.
• Targeted lint, server typecheck, formatting, and diff checks passed.
• Backend regression tests demonstrate the before/after behavior; screenshots are not applicable.

Authored with Codex.

[macroscopeapp]

Effect Service Conventions found a blocking error-boundary violation.

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

Approvability

Verdict: Approved at 1388b45

Macroscope's review found this PR approvable — This is a narrowly scoped server bug fix that improves messages for existing provider failure paths without changing workflows, defaults, schemas, or deployment behavior. The new cause handling is bounded and covered by targeted tests.

^{You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.1 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.4 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	1	—	8	✅
Claude	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	20.8 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	2	—	8	✅

Baseline: unavailable · PR result: 1388b45 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 106.1 KiB
• Claude decoded thread snapshot: 106.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.