agent: add remaining-spend helper and harden amount conversion

[karagozemin]

usdcToStroops currently accepts negative and non-finite amounts, silently producing invalid stroop values. Harden it to reject non-finite/negative input.

Also add a remainingAppraisalSpend helper so the bidder can report its remaining appraisal budget against the mandate cap.

[DeveloperEmmy]

Hello and good day. I'm experienced and would love to resolve your issues in no time. Thank you

[Olasunkanmi975]

Please assign

[grantfox-oss]

🦊 GrantFox — @Olasunkanmi975 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #193)
2. Your PR will be reviewed by the Sub-Rosa-Issue maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@Olasunkanmi975's PR #228 was approved and merged by @karagozemin.

🏆 @Olasunkanmi975: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (799 total points). Track your full progress on GrantFox.

👏 Great work, @Olasunkanmi975! Keep contributing to Sub-Rosa-Issue.