agent: enforce mandate spending caps with pre-flight balance check

[karagozemin]

Summary

The multi-agent bidder in services/agent should verify a bid stays within its mandate cap and that the account has sufficient balance before submitting.

Motivation

An agent exceeding its mandate or bidding without funds wastes fees and can strand a round.

Acceptance criteria

• Pre-flight check comparing bid + escrow against mandate cap and balance.
• Clear typed error when a bid would exceed the cap.
• Tests for within-cap, over-cap, and insufficient-balance cases.
• Behavior documented.

[Heazzy500]

Hello, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[Olasunkanmi975]

Please assign boss