Phase 2.8: Wire up stubbed batch/snapshot RPC methods

[aesslinger]

Part of the Phase 2 set of PostgreSQL-specific features beyond the built-in driver.

Problem

Four RPC methods are already registered in rpc.rs's dispatch table but stubbed with not_implemented() in src/handlers/metadata.rs, returning a -32601 error instead of real data:

• get_schema_snapshot
• get_all_columns_batch
• get_all_foreign_keys_batch
• get_materialized_view_definition

Found during a README audit (2026-08-18) and surfaced in the README's "Known Limitations" section pointing back here.

Why this differs from the other Phase 2 items

Every other Phase 2 issue (#25–#31) is new-feature design work — deciding what the feature should even look like. This is different: the RPC surface and method names already exist in the dispatch table, so this is implementation work against an already-agreed contract, not feature design.

Proposed approach

• get_materialized_view_definition — low-effort. Same pattern as the existing get_view_definition, reading from pg_matviews.definition.
• get_all_columns_batch, get_all_foreign_keys_batch, get_schema_snapshot — need a host-side caller check first. There's no existing caller in this repo to infer the expected response shape from, so confirm what tabularis's frontend actually expects (likely a batched/aggregated response across multiple tables, given the naming) before implementing — check tabularis's frontend source for any code that calls these method names, or ask the maintainers if the contract isn't otherwise discoverable.

Acceptance criteria

• All 4 methods return real data instead of -32601 errors
• get_materialized_view_definition matches get_view_definition's existing response shape/conventions
• The two *_batch methods and get_schema_snapshot match whatever shape the frontend caller actually expects (verified against real caller code, not guessed)
• README's "Known Limitations" section is removed/updated once all 4 land
• No regressions in existing Phase 1 parity tests

References

• Priority: Sprint 7 — low effort, already-agreed contract