feat(providers): support custom HTTP headers

[elliottophellia]

Summary

• validate, persist, protect, and forward optional headers for custom providers across dashboard, web onboarding, and terminal onboarding
• treat header-authenticated custom providers as setup-ready and eligible for model discovery while preserving adapter header precedence
• add the shared multiline editor/parser plus regression coverage for malformed headers, terminal flows, readiness, and authorization precedence

Verification

• go test ./internal/config ./internal/server ./internal/agent ./internal/llm ./cmd/antares
• bun test
• bun x tsc --noEmit --incremental false
• isolated dashboard, web-onboarding, and terminal-onboarding smoke flows against loopback OpenAI-compatible fixtures, including SSE chat completion

[enowdev]

Fixed and merged, including corrective commit 5522044. During review, a new named custom provider could inherit headers (including Authorization) from the legacy custom slot when headers were omitted. Removed that cross-provider fallback and prevented custom setup probes from forwarding the legacy headers. Explicit headers still work, and the legacy configuration is left untouched. Added regression coverage for omitted headers, explicit replacement headers, and actual probe requests. Local config/server/agent/LLM/CLI tests plus GitHub go, web, and smoke checks passed. Closed by merge after applying the correction; not discarded.