fix: guard milestone dispute status transitions

[maixuancanh]

Summary

• allow milestones to enter dispute only from Pending or InProgress
• reject disputes for achieved, missed, and already disputed milestones
• update the existing dispute flow tests and add regression coverage for each guarded status

Testing

• cargo test -p pulsar-milestone-tracker (20 passed)
• cargo fmt -p pulsar-milestone-tracker -- --check
• cargo clippy -p pulsar-milestone-tracker --all-targets
• cargo build -p pulsar-milestone-tracker --release --target wasm32-unknown-unknown
• cargo build --workspace

CI context

The two failing repository-wide checks are also failing on main in run 30295342591:

• Frontend CI reports the same existing lint backlog (68 errors and 20 warnings).
• Contracts CI resolves ed25519-dalek 3.0.0 for soroban-env-host 22.1.3 and fails on their incompatible CryptoRng traits before compiling workspace tests.

With ed25519-dalek resolved to 2.2.0 locally, cargo test --workspace proceeds past that dependency and reaches separate pre-existing compile errors in contracts/publisher-verification/src/test.rs. The focused milestone-tracker suite passes in full.

Closes #710

[drips-wave]

@maixuancanh Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits