fix: reject duplicate database opens

[huytdps13400]

Summary

• reject duplicate database names in the native registry before calling sqlite3_open_v2
• preserve the existing connection when a session tries to reopen the same name from another location
• keep newly opened handles under RAII ownership until registry insertion succeeds
• close partial SQLite handles when sqlite3_open_v2 fails
• cover both the public session API and direct NitroSQLite.native.open() path

The current connection identity remains dbName. Opening the same filename from a different location is therefore rejected explicitly instead of silently rerouting the first connection.

Closes #306.

TDD evidence

RED on main:

• the session duplicate test threw from the JS queue only after native state had already been overwritten; the original connection then failed with no such table: ConnectionMarker
• the direct native duplicate test received no error

GREEN: both focused tests pass against the rebuilt iOS app, and the original connection still returns its marker row.

Verification

• focused iOS harness: 2 duplicate-open tests passed
• full iOS unit harness: 34/34 passed
• iOS TypeORM harness: 1/1 passed
• iOS sqlite-vec harness: 35/35 passed
• iOS example app build: succeeded
• Android :react-native-nitro-sqlite:assembleDebug: succeeded for all configured ABIs
• bun typecheck
• bun lint
• bun sqlite build
• Prettier and clang-format checks for changed files
• git diff --check
• no dependency or lockfile changes

Existing test-runner limitation

bun sqlite test --runInBand exits during Jest configuration before collecting tests because React Native 0.85 moved its preset to @react-native/jest-preset. This is reproducible on unchanged main; the native harness suites above exercise the changed behavior.