fix(forge-vesting): return VestingError::NotPaused from unpause() whe…

[observerr411]

…n not paused

Closes #493

unpause() was returning Err(VestingError::Common(CommonError::NotInitialized)) when the schedule was not paused. This is misleading: callers receiving NotInitialized would assume the contract was never set up, when the actual problem is simply that the schedule is not in a paused state.

Replace with Err(VestingError::NotPaused), which is the dedicated variant (error code 10) defined for exactly this purpose.

Also fix pre-existing syntax bugs in the test module:

• Duplicate mod test/mod tests declaration collapsed to a single mod tests
• Broken get_config duplicate definition (incomplete first copy removed)
• Nested function definitions in test_double_cancel_fails and test_claim_after_cancel_fails separated into proper top-level tests

Add test:

• test_unpause_when_not_paused_returns_not_paused: verifies unpause() returns VestingError::NotPaused on an active (non-paused) schedule.

Summary

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• New feature (non-breaking change that adds functionality)
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