Enforce minimum task bounty

[kitWarse]

Summary

• Adds a global, admin-configurable minimum bounty for new task registrations.
• Enforces the minimum against TaskConfig.gas_balance before a task ID is allocated, so rejected low-bounty registrations do not consume IDs.
• Keeps backwards compatibility by defaulting the minimum bounty to 0 until an admin configures it.

Issue

• Closes Contract: Implement minimum bounty validation #696

Changes

• Added MinBounty contract storage and public get_min_bounty / set_min_bounty contract methods.
• Added admin authorization for minimum bounty updates using the configured AdminAddress / proxy admin path.
• Added BountyBelowMinimum and InvalidBounty contract errors.
• Updated register to revert when gas_balance is below the configured minimum bounty.
• Added access-control and registration tests for authorized updates, unauthorized updates, below-minimum rejection, and at-minimum acceptance.

Validation

• Passed: git diff --check
• Not run: cargo fmt / contract tests because cargo is not installed or available in PATH in this local environment.

Notes

• The minimum bounty defaults to 0, so existing deployments preserve current registration behavior until an admin sets a higher threshold.

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