Vault: explicit `panic!` removal sweep — replace any remaining string panics with `VaultError`

[greatest0fallt1me]

Description

We migrated most panics to VaultError but a few sites still call panic!("...") (notably in the withdraw_to path and some metadata setters). Sweep contracts/vault/src/lib.rs for any remaining panic! invocations and replace with the appropriate VaultError variant, adding new ones if needed.

Requirements and Context

• Inventory every panic! and unwrap site under contracts/vault/src/lib.rs
• Replace with typed errors; add VaultError variants where none fit
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b audit/vault-panic-sweep

2. Implement changes
   • contracts/vault/src/lib.rs
   • contracts/vault/src/test.rs (assert new codes)
3. Test and commit
   • cargo test -p callora-vault
   • Cover edge cases: every replaced path tested
   • Include test output and notes in the PR

Example commit message

audit(vault): replace remaining string panics with VaultError variants

Acceptance Criteria

• Zero panic!( in production paths under contracts/vault/src/lib.rs
• New VaultError variants documented inline
• WASM size delta recorded

Guidelines

• .rs only under contracts/.../src/, NatSpec-style /// doc comments, 95% coverage
• Use real Soroban SDK idioms; no unwrap() in production paths
• Clear documentation and inline comments
• Timeframe: 96 hours

[Kingvic300]

@Kingvic300 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would love to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kingvic300 to this issue.

[grantfox-oss]

🦊 GrantFox — @Kingvic300 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #448)
2. Your PR will be reviewed by the CalloraOrg maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @Kingvic300! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Kingvic300: 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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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Kingvic300's PR #453 was approved and merged by @greatest0fallt1me.

🏆 @Kingvic300: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (83 total points). Track your full progress on GrantFox.

👏 Great work, @Kingvic300! Keep contributing to CalloraOrg.