feat: return signed log download URLs from harness_diagnose when return_download_url is set

[thisrohangupta]

Description

Ports the remaining changes from Saranya-jena/mcp-server#1 onto main.

harness_get for execution_log already supported return_download_url=true on main (via resolveLogDownloadUrl). This PR completes the feature by wiring the same opt-in through harness_diagnose pipeline diagnostics:

• When options.return_download_url=true, failed step logs and requested_step_log return { download_url: "<signed logs.zip URL>" } instead of inline/truncated log text.
• Agents can fetch and paginate logs locally without buffering large blobs in the MCP server process (avoids OOM on long executions).

Changes

• src/tools/diagnose/pipeline.ts: Add resolveDiagnoseLog helper; honor return_download_url for failed steps, explicit step requests, and auto-fetched success logs.
• src/tools/harness-diagnose.ts: Document return_download_url in pipeline diagnostic options.
• Tests: Coverage for diagnose pipeline, harness_get execution_log URL mode, and resolveLogDownloadUrl.

Type of Change

• New feature
• Bug fix
• Refactor
• Documentation

Checklist

• Tests pass (pnpm test — 1994 tests)
• Typecheck passes
• Build + docs check pass

Credit

Based on work by @Saranya-jena in Saranya-jena#1

  

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