aibridge: fix YAML config key naming inconsistency

[ssncferreira]

The AIBridge YAML configuration uses inconsistent naming conventions. Most keys use snake_case, but at least two use camelCase:

• maxConcurrency should be max_concurrency
• rateLimit should be rate_limit

Question: Should this be considered a breaking change? Do we need a migration phase (e.g., support both rateLimit and rate_limit)? Or since aibridge will transition from experimental, can we consider this part of that work?

Related to: coder/coder#21296 (comment)