feat: per-request timeout and retry overrides

[0xneobyte]

Problem

timeout and max_retries are configured once at client initialization and apply globally to every request. There is no way to override them for individual calls. A developer querying a complex document may need a 60s timeout for that one call without affecting all other requests.

Proposed Behaviour

Each method accepts optional parameters to override the global defaults for that call only.

# Long timeout for one specific call, global default unchanged
response = await client.query("...", timeout=60.0, max_retries=0)

• Overrides apply to that single call only
• Supported parameters: timeout, max_retries
• Global client config is never mutated

Files to Modify

File	Change
src/brainus_ai/client.py	Add optional timeout and max_retries kwargs to query(), get_usage(), get_plans()

Acceptance Criteria

• query() accepts timeout and max_retries overrides
• get_usage() and get_plans() accept the same overrides
• Global client config is not affected by per-request overrides
• Default behaviour when no overrides are passed is unchanged