feat(jdbc): add user-managed JDBCX support

[SpencerZhang]

What changed

• add JDBCX as a first-class JDBC connection profile with the official icon
• load a user-installed io.github.jdbcx:jdbcx-driver:<version> runtime together with all vendor JDBC drivers from Driver Store
• support JDBCX URLs when parsing and rewriting tunneled JDBC endpoints
• import JDBCX connections with the correct profile and wrapped driver class
• document the user-managed JDBCX and vendor-driver installation model

Why

JDBCX connections need both the JDBCX wrapper runtime and a database vendor driver. Keeping both in Driver Store avoids vendor-specific hardcoding and does not increase the DBX application or JDBC plugin package size.

User impact

Users install a JDBCX Maven bundle and the target database JDBC driver in Driver Store, then connect with URLs such as jdbcx:mysql://127.0.0.1:3306/test. DBX validates that the JDBCX runtime exists and supplies the complete driver-store classpath.

Validation

• pnpm exec vitest run apps/desktop/src/lib/__tests__/database/jdbcxBuiltinDriver.spec.ts apps/desktop/src/lib/__tests__/imports/jdbcxImport.spec.ts
• pnpm typecheck
• pnpm build
• mvn -q test in plugins/jdbc
• cargo test -p dbx-core jdbc --lib (51 passed)