fix: honor configured deadlines for Agent streams

[grantjayy]

Problem

Agent requests have an absolute PERPLEXITY_TIMEOUT_MS deadline, but native fetch and default ProxyAgent inherit Undici's independent 300000ms headers/body-idle limits. A valid quiet stream can fail before a longer configured deadline.

Streaming errors also discard error.cause.code, making body timeouts and peer socket loss indistinguishable. Both reproduce TypeError: terminated. A historical 401-second interruption cannot be attributed conclusively because that evidence was discarded.

Solution

Give each Agent call a dispatcher whose headers/body limits match the application deadline. Destroy that dispatcher after timer/listener cleanup. Retain only allowlisted UND_ERR_* cause codes in streaming errors.

Scope

Only Agent transport configuration and regression tests change. Public tools, schemas, presets, proxy precedence, search/cancel behavior and connect timeout remain unchanged. No retry, background API migration, new dependency, global dispatcher change, or version bump.

Validation

• npm test: 105/105 pass, five files, no skipped tests.
• npm run build: pass.
• npm pack --dry-run: no compiled tests in package.
• New wire tests against unchanged base: eight expected defect failures; seven guards pass. After correction, all 15 pass.
• Packaged stdio checks on Node24 and actual Node22: delayed headers, silent body, socket loss, and deadline cancellation pass. Each call submits one Agent POST; timeout submits one cancel.
• Heavy independent review: two first-wave reviewers; one confirmed missing search network-error test repaired. Fresh incremental review passes with no unresolved medium+ findings.

Evidence and context

Related prior reports: #51 (transport timeout surviving application timeout), #110/#122 (MCP client progress), #83/#85 (old Sonar streaming). This correction addresses the current Agent transport path, not those older paths.

Base: c73c8561bbc2d9eb666334a53c311b50f4f4cf76. Reviewed head: e2ce829ba490f0fd190e8cdea78588f79018c2d6.

Local untracked plan: ~/.hermes/plans/modelcontextprotocol/2026-09-16-agent-stream-termination.md; canonical SHA256 5ad3982b8995c014ccd9ac5359dc6026bf809e369c9b01e2841aeb525e46f380 (declared digest token replaced with PENDING).

Risk

Genuine provider/socket interruptions remain errors; accepted requests are never blindly resubmitted. Per-call dispatchers sacrifice cross-call keep-alive reuse for explicit lifecycle and independently configured deadlines.

Notes / accepted debt

• Invalid proxy settings can now return bare Invalid URL rather than the previous prefixed error; still fail closed.
• Existing malformed-timeout warning remains unchanged.
• Fixture assertions use real HTTP and CONNECT paths with shortened inherited defaults; some ancillary timing/count assertions are covered by separate packaged acceptance.
• Existing search/cancel proxy lifecycle remains outside this Agent-only change.
• Destroy-error masking was rejected as a false positive after inspection of Undici's resolving destroy implementation.
• Cancel interference was rejected: cancel uses an independent dispatcher and the wire checks record the cancel.

Attribution and delivery

Standard implementation: GJC investigative/direct. Test-only repair: GJC procedural/direct. Heavy review: Claude Fable 5.1, high effort. One repair round. Human-facing PR text: Oren.

Open and unmerged. No upstream merge requested.