Replace unchecked arithmetic in extend_milestone_deadline with checked operations

[francisdouglas-ux]

Problem & Goal

Convert the arithmetic inside extend_milestone_deadline to checked operations that return a typed error, so no input can cause a wrap or an unhandled panic.

Context & Components

• Target Repository: escrow-contract
• Module/Component: extend_milestone_deadline (Extends the auto-release deadline of a milestone)

Implementation Details

1. Review the existing codebase inside the escrow-contract directory.
2. Locate the source files relating to extend_milestone_deadline.
3. Implement the feature logic/validation rules step-by-step.
4. Ensure code conforms to codebase patterns (lints, formats, types).

Verification & Testing Requirements

• Validation check: Assert i128::MAX and i128::MIN operands return Error::InvalidAmount rather than panicking, and that valid amounts produce identical results to before.
• Add comprehensive test cases matching these validation requirements.
• Ensure all other existing project tests continue to compile and pass.

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Note: This issue is completely self-contained and does not block or require any other issues to be resolved.

[Awesome254]

@Awesome254 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer Good day
I'm interested and would love to work on this issue as soon as it is assigned to me
Thanks

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[CodeNinjaStephen]

@CodeNinjaStephen has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer, I’ll appreciate the opportunity to work on this issue

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[qa-eden]

@qa-eden has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer, I would work on this

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[DevFaster]

@DevFaster has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hi repo maintainer. please assign me this issue . i will fix this issue correctly.

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[onyemaechiezekiel9]

@onyemaechiezekiel9 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello boss, I've gone through the issue description and i'd love to work on this issue, kindly assign me

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