Fix duplicate JDBC connection parameters

[sreekanth-db]

Description

• Allow a connection parameter to be supplied in both the JDBC URL and Properties without failing on a duplicate map key.
• Preserve JDBC URL precedence when duplicate values differ.
• Add regression coverage for identical and conflicting values with case-insensitive parameter names.

Fixes #1648

Testing

• DatabricksConnectionContextTest: 147 tests passed.
• Live warehouse repro confirmed that duplicate parameters connect successfully and the JDBC URL value takes precedence.
• isaac review --uncommitted: 0 final findings.

Telemetry Errors

• Not applicable — this PR does not add or change a telemetry-visible error.
• Applicable — the error uses DatabricksDriverErrorCode where appropriate, and any new code is uniquely numbered and tested.
• Applicable — its driver/server/user classification is linked, or maintainer help is requested because the author cannot access the classification.

Additional Notes to the Reviewer

The precedence behavior is documented in the helper method and the changelog.

[peco-review-bot]

✅ No issues identified by the review bot.