(feat): implement grant contract with flow rate management and error …

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This pull request introduces a major refactor of the GrantContract to implement streaming/linear vesting grants with flow rates, replacing the previous milestone-based grant logic. The contract now supports dynamic updates to grant flow rates, cancellation, withdrawal, and status tracking, and includes a comprehensive suite of tests for these new behaviors. The previous milestone logic and associated tests have been removed. Below are the most important changes:

Core Contract Refactor and New Features

• Replaced milestone-based grant logic with a streaming/flow-rate-based grant system, introducing the Grant, GrantStatus, and related data structures. Grants now accrue claimable balances over time according to a flow rate, and can be updated, cancelled, or completed. (contracts/grant_contracts/src/lib.rs)
• Implemented core contract methods: initialize, create_grant, cancel_grant, get_grant, claimable, withdraw, and update_rate, each with appropriate authorization and error handling. (contracts/grant_contracts/src/lib.rs)
• Added robust error handling and validation for grant operations, including checks for authorization, invalid states, math overflows, and invalid parameters. (contracts/grant_contracts/src/lib.rs)

Testing Overhaul

• Replaced all milestone-based tests with new tests covering flow-rate grants: updating rates (including edge cases), withdrawals, pausing/resuming, admin authorization, and status transitions (active, cancelled, completed). (contracts/grant_contracts/src/test.rs)

Other

• Removed the old grant math utility and milestone logic, as well as the associated simulation and overflow tests, since they are no longer relevant to the new flow-rate model. (contracts/grant_contracts/src/lib.rs, contracts/grant_contracts/src/test.rs) [1] [2]
• Updated the Rust toolchain info and added a note about a lock file parse error (likely unrelated to contract logic). (target/.rustc_info.json, target/flycheck0/stderr) [1] [2]
  Closes # Issue: [Logic] Dynamic Flow Rate Adjustment #6