[vibeinging]

A connection opened with access_mode=DSQL_MODE_READ_ONLY currently accepts the setting but reports mode 0 and permits a zero-row DELETE. current_catalog also returns an empty string instead of the connected database name, causing downstream connection validation to fail.

This change applies read-only mode before login succeeds, keeps that session pinned across autocommit changes, invalidates prepared bindings when the access mode changes, and closes the connection if read-only protection cannot be restored. The catalog getter now reads the actual database name and propagates query errors.

Validation on native macOS ARM64 / CPython 3.12 against a real DM8 instance:

• 20 integration tests passed with no skips, including 7 new access-mode/catalog regressions and neighboring connection/cursor contracts.
• All 7 new regressions fail against the original bridge.
• An unmodified official 2.5.38 C extension using the patched bridge passed downstream connection testing, schema discovery, parameterized read-only queries, 8 concurrent queries, write rejection and invalid-password recovery. Source-table snapshots were unchanged.

The test report records the tested combinations and the separate pre-existing executemany limitation with the official C extension. This PR contains source and tests only; it does not distribute a vendor DPI SDK or compiled driver artifacts.