Standardize versioned event schema for payment, enrollment, governance, and role actions

[Junirezz]

Closes #918

Summary

Events across the payment, enrollment, governance, and role-management domains were ad hoc: raw Address/Symbol values stuffed directly into topics, no schema version, no timestamps, and no test ever asserted on event contents. There was also an existing, well-built versioned EventPublisher module (contracts/src/events.rs) that nothing in the codebase actually called.

Every event now follows one shape, documented at the top of events.rs:

topics: (event_name: Symbol, schema_version: Symbol, ..indexed fields)
data:   a single #[contracttype] struct with the full field list (actors, amounts, timestamps)

schema_version only bumps on a breaking payload change (field removed/renamed/retyped) — adding a field is non-breaking. This lets indexers detect incompatible changes instead of guessing from payload shape.

Wired up:

• payment_gateway.rs: payment_processed / payment_released / payment_refunded / payment_disputed / dispute_resolved
• enrollment.rs: student_enrolled / enrollment_completed / enrollment_dropped
• governance.rs: gov_initialized / credits_deposited / proposal_created / vote_cast / proposal_finalized / proposal_executed
• rbac.rs: rbac_role_granted (previously not emitted at all for init_rbac or grant_role) / rbac_role_revoked

Also fixes two pre-existing bugs found while adding coverage:

• get_default_roles() in rbac.rs built role data with its own internal Env::default() instead of the contract's real Env, so init_rbac() failed on every call.
• governance_test.rs (18 tests) existed on disk but was never wired into the module tree, so it silently never compiled or ran.

Known gap, explicitly left out of scope: certificate-lifecycle events already have a complete, well-designed schema in events.rs (Minted/Revoked/Renewed/BatchMinted/etc.), but the certificate-minting contract that would call it could not be located as compiled/reachable code in this snapshot — the only reference is contracts/src/tests.rs, which is itself unwired and references functions (e.g. batch_issue) that don't exist anywhere in the crate. Wiring that up is a separate, larger undertaking and is flagged here rather than guessed at.

Test plan

• New event-content assertion tests (env.events().all()) added for each domain — none existed before
• cargo test in contracts/ — all 277 tests pass
• cargo fmt --check / cargo clippy clean

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[ayomideadeniran]

Pr under review