Fix const assignment warnings

[adamdickmeiss]

No description provided.

[Copilot]

Pull request overview

This PR aims to eliminate compiler warnings about assigning/modifying const data by tightening pointer constness and avoiding in-place string edits across a few parsing/caching paths.

Changes:

• Reordered includes to keep config.h first in a test file.
• Switched a MARC SAX test away from xmlSAXUserParseMemory to a push-parser context to avoid const-related warnings.
• Reworked several string-splitting paths (SOLR scan term splitting, diagnostic URI parsing, MARC line subfield handling) to avoid modifying source buffers.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/test_tpath.c	Moves includes below config.h to match typical include ordering.
test/test_marc_read_sax.c	Replaces in-memory SAX parsing with a libxml2 push parser context.
src/zoom-record-cache.c	Refactors diagnostic URI parsing to avoid in-place edits (but currently introduces a crash/logic bug).
src/solr.c	Avoids mutating facet term strings by using length-bounded duplication.
src/retrieval.c	Updates temporary pointers to const char * where appropriate.
src/marc_read_line.c	Attempts to avoid const warnings while removing a blank after subfield markers (but currently corrupts output).

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