Strengthen base URI transaction regression test

[hmottestad]

GitHub issue resolved: #5403

Briefly describe the changes proposed in this PR:

---

PR Author Checklist (see the contributor guidelines for more details):

• my pull request is self-contained
• I've added tests for the changes I made
• I've applied code formatting (you can use mvn process-resources to format from the command line)
• I've squashed my commits where necessary
• every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change

[Copilot]

Pull Request Overview

This PR strengthens regression testing for base URI handling in transactions by adding comprehensive test coverage and fixing a bug in base URI parameter encoding. The changes ensure that base URIs are passed as plain strings rather than being incorrectly encoded as RDF values.

• Added integration test to verify base URI resolution works correctly end-to-end
• Added unit test to verify base URI parameter is passed correctly in HTTP requests
• Fixed bug where base URI was being encoded as an RDF IRI instead of passed as a plain string

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
RemoteRepositoryBaseUriIT.java	New integration test verifying base URI resolution with RDF/XML data
RDF4JProtocolSessionTest.java	New unit test verifying base URI parameter handling in HTTP requests
RDF4JProtocolSession.java	Bug fix to pass base URI as plain string instead of encoded RDF value

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

[nitpick] Consider extracting the trimmed base URI validation into a separate method or using a guard clause to reduce nesting. The current nested if structure could be flattened for better readability.

Suggested change

	if (baseURI != null) {
	String trimmedBaseURI = baseURI.trim();
	if (!trimmedBaseURI.isEmpty()) {
	url.setParameter(Protocol.BASEURI_PARAM_NAME, trimmedBaseURI);
	}
	}
	setBaseURIParameter(url, baseURI);