feat: make MAX_SPONSORS configurable per deployment (#96)

[rafaio1]

Summary

Resolves #96

This PR makes MAX_SPONSORS a per-deployment configuration parameter instead of a hardcoded compile-time constant, enabling different escrow/milestone instances to support varying numbers of sponsors based on their specific use case (e.g., $50 single-issue bounties vs $50k release milestones).

Changes

• escrow/src/lib.rs: Added optional max_sponsors: Option<u32> parameter to initialize(). Stores value in instance storage under DataKey::MaxSponsors. Defaults to 20 (DEFAULT_MAX_SPONSORS) when None is passed for backward compatibility. Rejects zero with InvalidAmount.
• milestones/src/lib.rs: Identical changes as escrow contract.
• escrow/src/types.rs & milestones/src/types.rs: Added MaxSponsors variant to DataKey enum.
• Tests: Updated all initialize/try_initialize calls in both contracts' test suites to pass &None for the new parameter. Replaced crate::MAX_SPONSORS references with crate::DEFAULT_MAX_SPONSORS.

Testing

• ✅ cargo test --manifest-path contracts/escrow/Cargo.toml — 36 tests passed
• ✅ cargo test --manifest-path contracts/milestones/Cargo.toml — 20 tests passed
• ✅ cargo build succeeds for both contracts

Backward Compatibility

Existing deployments are unaffected. New deployments that omit the max_sponsors parameter will default to 20, matching current behavior. The initialize signature change is breaking for direct callers but this is expected for a pre-mainnet contract.

[vercel]

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[rafaio1]

CI Note

The Vercel deployment check failure is expected and unrelated to this PR's code changes. It requires team authorization for the chonilius' projects Vercel team, which is a repository configuration matter.

This PR modifies only Soroban smart contract Rust code (contracts/escrow and contracts/milestones). There are no frontend or web assets changed. The relevant validation is cargo test and cargo build, both of which pass locally:

• ✅ cargo test --manifest-path contracts/escrow/Cargo.toml — 36/36 passed
• ✅ cargo test --manifest-path contracts/milestones/Cargo.toml — 20/20 passed
• ✅ cargo build succeeds for both contracts

If there's a separate Rust CI workflow configured for this repo, I'm happy to address any feedback from it. Otherwise, the code changes are ready for review.