Add public contract config getter

[es3298]

Closes #202.

Summary

• Add a stable ContractConfig return type with fee config, treasury, admin count, pause state, version, and public limits.
• Add public get_config getter for frontends/indexers.
• Return None for fee config/treasury before those settings are initialized, so the getter works immediately after initialize.
• Add focused tests for post-initialization defaults and updated fee configuration.
• Document get_config in the contract README and interface guide.

Validation

• git diff --check

Additional validation notes:

• rustfmt dongle-smartcontract/src/lib.rs dongle-smartcontract/src/types.rs dongle-smartcontract/src/tests/mod.rs dongle-smartcontract/src/tests/config.rs is currently blocked by the repository's existing dongle-smartcontract/src/utils.rs unclosed-delimiter parser error.
• cargo test --manifest-path dongle-smartcontract\Cargo.toml config -- --nocapture did not complete before the local timeout in this checkout; the existing parser/compile baseline is already called out in Add batch TTL extension endpoints #305.

[es3298]

@Naomi-Gift this PR is ready for GrantFox review for #202. It is open and mergeable, adds the public get_config snapshot with fee config, treasury, admin count, pause state, version, and public limits, plus tests and docs. Happy to provide the GrantFox/Stellar payout destination privately after acceptance/merge.