config: reject non-positive periodic intervals before ticker creation

Summary

Validate every ticker-driving duration before runtime construction and make periodic start/close lifecycle idempotent. Owner surfaces: src/semantic-router/pkg/selection/elo.go, src/semantic-router/pkg/selection/rl_driven.go, src/semantic-router/pkg/selection/lookuptable/file.go, src/semantic-router/pkg/extproc/router_selection.go, and src/semantic-router/pkg/config/validator_learning.go.

Current behavior

Syntactically valid zero or negative durations can reach ticker construction and panic the process or leave periodic behavior silently inactive. At least one storage close path also uses a check-then-close pattern that is unsafe under concurrent close.

Public API boundary

The listed Elo, RL-driven, and lookup-table files are legacy/internal cleanup surfaces on current main. Fixing their ticker/lifecycle safety must not reintroduce their historical names as supported public Router Learning configuration; #2242 owns that migration boundary.

Expected behavior

Every periodic duration is positive and bounded at config ingestion and constructor boundaries, and start/close behavior is concurrency-safe and idempotent.

Acceptance

• Require every ticker-driving duration to be positive and below a documented maximum.
• Return a field-addressed config error before runtime construction.
• Defensively reject invalid intervals in constructors.
• Define and synchronize start, closing, and closed transitions.
• Make repeated and concurrent close safe.

Validation

• Add table tests for zero, negative, overflow, malformed, and maximum-boundary durations.
• Add repeated-start and high-concurrency close tests under -race.
• Run cd src/semantic-router && go test -race ./pkg/selection ./pkg/selection/lookuptable ./pkg/extproc ./pkg/config.
• Run the selection changed-surface gates reported by make agent-report.

Related

• Parent audit: #2375