## Description
Adds support for configuring multiple API keys per AI Bridge provider. This PR introduces the configuration parsing and validation only; wiring the key pools into the aibridge providers will happen in upstream PRs.
## Changes
Providers now accept a comma-separated list of keys via the `KEYS` env var (or a single key via the existing `KEY` var). The two are mutually exclusive. Bedrock follows the same pattern with `BEDROCK_ACCESS_KEYS` / `BEDROCK_ACCESS_KEY_SECRETS`, with an additional validation that the two slices have matching lengths.
Key validation at startup checks for empty values, duplicates, and a maximum of 5 keys per provider.
Related to: https://github.com/coder/internal/issues/1445
> [!NOTE]
> Initially generated by Coder Agents, modified and reviewed by @ssncferreira
Susana Ferreira
101a4082dd