fix(control-plane): implement graceful shutdown on SIGTERM/SIGINT

[7vignesh]

Summary

Replace the select{} no-op with real signal handling and HTTP server drain so SIGTERM during rolling deploys no longer kills in-flight requests. Background goroutines (presence manager, health monitor, cleanup service, OTel tracer, etc.) are now stopped cleanly before the process exits.

Changes:

• Install signal.NotifyContext for SIGINT/SIGTERM in runServer
• Replace gin Router.Run() with net/http.Server for Shutdown() support
• Call server.Stop() on signal: drains HTTP connections, stops background goroutines
• Add configurable shutdown_timeout (default 30s) via YAML and AGENTFIELD_SHUTDOWN_TIMEOUT env var
• Remove // TODO: Implement graceful shutdown comments
• Add nil guard for healthMonitor.Stop() to prevent panic on empty server

Closes #427

Type of change

• Bug fix

Test plan

• cd control-plane && go test ./cmd/agentfield-server/ -v -run "TestLoadConfig|TestRunServer"
• cd control-plane && go test ./internal/config/ -count=1
• cd control-plane && CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build ./cmd/agentfield-server
• cd control-plane && CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 go build ./cmd/agentfield-server
• Manual: started server locally, sent SIGINT (Ctrl+C), confirmed "Shutdown signal received, draining connections..." and clean exit with code 0

Test coverage

• I ran tests for the surface(s) I changed locally.
• New code paths are covered by tests in this PR (no bare additions).
• If I removed code, I updated coverage-baseline.json in this PR only if the removal caused a legitimate regression and I called it out in the summary above.
• The coverage gate check is green in CI before requesting review.

Checklist

• I have read CONTRIBUTING.md (if present) and docs/DEVELOPMENT.md.
• Commits are signed and follow conventional-commits style.
• I have linked any related issues (Fixes [Control Plane] Graceful shutdown not implemented (literal TODO) #427).

Related issues / PRs

Fixes #427

[Copilot]

Pull request overview

Implements first-class graceful shutdown for the control-plane server: replaces the previous “block forever” behavior with SIGINT/SIGTERM handling and introduces an http.Server so in-flight HTTP requests can be drained during rolling deploys.

Changes:

• Added signal.NotifyContext-based shutdown waiting in runServer, followed by a server.Stop() shutdown sequence.
• Replaced gin.Engine.Run() with net/http.Server.ListenAndServe() and added http.Server.Shutdown() on stop.
• Introduced configurable agentfield.shutdown_timeout (default 30s) with an AGENTFIELD_SHUTDOWN_TIMEOUT env override.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
control-plane/internal/server/server.go	Switches HTTP startup to net/http.Server and adds HTTP shutdown behavior to Stop().
control-plane/internal/config/config.go	Adds ShutdownTimeout to config, sets defaults, and supports env override.
control-plane/config/agentfield.yaml	Documents/configures default shutdown_timeout: 30s.
control-plane/cmd/agentfield-server/main.go	Adds SIGINT/SIGTERM waiting and triggers Stop() for graceful shutdown.

---

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[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	86.90%	87.40%	↓ -0.50 pp	🟡
sdk-go	91.80%	92.00%	↓ -0.20 pp	🟢
sdk-python	93.76%	93.73%	↑ +0.03 pp	🟢
sdk-typescript	90.09%	90.42%	↓ -0.33 pp	🟢
web-ui	84.76%	84.79%	↓ -0.03 pp	🟡
aggregate	85.55%	85.75%	↓ -0.20 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	77	89.00%	✅
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.

[santoshkumarradha]

Thanks for the graceful shutdown fix. I pushed a small follow-up commit that drains HTTP first, keeps the HTTP server pointer behind a mutex, wraps the startup error, and adds the invalid AGENTFIELD_SHUTDOWN_TIMEOUT env test.

I also resolved the Copilot threads after checking the changes. Local checks passed:

• go test ./internal/config/ -count=1
• go test ./internal/server/ -count=1
• go test -race ./internal/server/ -run "TestStopGracefulShutdown|TestStopHTTPServerShutdownTimeout" -count=1
• go test ./cmd/agentfield-server/ -v -run "TestLoadConfig|TestRunServer" -count=1

[santoshkumarradha]

Thanks for the follow-through here. I rechecked the graceful shutdown path, the config surface, and the added regression coverage. The shutdown timeout is wired through YAML and env overrides, the HTTP server now drains before process exit, and the required checks plus CLA are green. This looks good to merge.