Harden caller authorization and precondition guards in apply_dispute_arbitration_split

[godamongstmen897]

Problem & Goal

Add the authorization and precondition checks at the top of apply_dispute_arbitration_split so the function rejects unauthorized callers and illegal source states before it reads or writes any ledger entry.

Context & Components

• Target Repository: escrow-contract
• Module/Component: apply_dispute_arbitration_split (Applies an arbiter-decided client/freelancer split)

Implementation Details

1. Review the existing codebase inside the escrow-contract directory.
2. Locate the source files relating to apply_dispute_arbitration_split.
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 an unauthorized caller and an illegal source state each return their specific typed error, and that no storage entry is mutated in either case.
• 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.

[Xoft31]

@Xoft31 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I understand the expected outcome and would love to work on this. I'll provide a solution that satisfies the acceptance criteria.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Xoft31 to this issue.

[nice-bills]

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

Hello Maintainer, I’ve assessed the task and believe my experience is relevant to the problem. Kindly allow me to work on it.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @nice-bills to this issue.

[drips-wave]

Congratulations, @nice-bills! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @nice-bills: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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