test(coverage): drive pool/circuit/telemetry/ctxkeys/root to ≥95%

[mastermanas805]

Summary

Drives the provisioner's POOL + ROOT + CIRCUIT (plus telemetry/ctxkeys) packages to ≥95% statement coverage. Scope deliberately excludes backend/postgres, backend/mongo, backend/queue, backend/redis, server, and handlers (owned by other agents).

package	before	after
internal/pool	26.3%	96.4%
internal/circuit	94.9%	100.0%
internal/telemetry	80.4%	98.3%
internal/ctxkeys	no tests	test file added (pkg has no statements)
root (main Run seam)	32.1%	96.1%

What's covered

• pool: full DB-integration lifecycle (migrate/Start/Claim/Stats/fillPool/provisionOneItem), per-type backend arms, failing-backend + closed-pool error paths, periodic-tick refill.
• circuit: constructor clamps, isCallerDeadline(nil), half-open→reopen onOpen callback.
• telemetry: plaintext/TLS endpoint arms, NR-license header arm, exporter/resource construction-error arms.
• root: extracted run()/bootstrap()/realMain() + signalContext()/netListen() seams from main so boot→ready→teardown, the os.Exit-class error arms, the pool-enabled path, and the gRPC serve-error arm are all testable without spawning a process or real signals.

Production changes

Behavior-preserving indirection seams only:

• pool.Manager.tickInterval (defaults to 30s — prod unchanged)
• telemetry.newExporter/newResource wrap the real OTel ctors
• main delegates to realMain; netListen = net.Listen

Test plan

• internal/circuit 100%, internal/telemetry 98.3%, internal/pool 96.4%, root 96.1%
• Full make gate equivalent (build + vet + go test ./... -short -p 1) green — all 17 packages pass, no regressions
• DB-touching tests gate on TEST_PROVISIONER_DATABASE_URL and skip cleanly when unset

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