feat(java/driver/jni): add fluent connection interface

[lidavidm]

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[Copilot]

Pull request overview

This PR introduces a fluent builder-style API on JniDriver to simplify configuring JNI driver-manager parameters and opening an AdbcDatabase, and updates JNI validation integration tests to exercise the new fluent path.

Changes:

• Add JniDriver.load() and an inner Builder with fluent setters (driver, profile, uri, param) and open().
• Update SQL Server integration test to also validate the fluent API path.
• Update PostgreSQL integration setup to use the fluent API and clean up unused imports.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
java/driver/jni/src/main/java/org/apache/arrow/adbc/driver/jni/JniDriver.java	Adds a fluent builder API for configuring parameters and opening a JNI-backed database.
java/driver/jni-validation/src/test/java/org/apache/arrow/adbc/driver/jni/SqlServerIntegrationTest.java	Adds coverage for opening a DB via the new fluent API.
java/driver/jni-validation/src/test/java/org/apache/arrow/adbc/driver/jni/PostgresIntegrationTest.java	Switches test setup to the fluent API and removes now-unused imports.

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