aibridge: key pool data structure and provider configuration

[ssncferreira]

Description

• KeyPool type with state management (valid, temporary, permanent)
• Selection strategy abstracted to allow future changes (e.g., round-robin)
• Concurrency: concurrent reads, mutually exclusive writes, lazy cooldown expiry
• Config parsing and startup validation in coder
• Wire key pool through providers to interceptors (no failover yet)
• Remove 429 from circuit breaker failure conditions (scope separation between key errors and provider errors). Note: we should probably add 429 handling for BYOK cases.