feat(indexing): add markdown document indexing

[shahzadarain]

Summary

Adds an index-markdown-documents MCP tool that indexes markdown content into Solr, alongside the existing JSON/CSV/XML tools — extracting searchable structure rather than a flat text blob.

Closes #69

Design

A new MarkdownDocumentCreator follows the existing strategy pattern in indexing/documentcreator/, parsing with CommonMark-Java 0.28.0 plus its YAML front matter extension (lightweight, reflection-free, so GraalVM native-image safe — no extra hints needed).

Field extraction:

Field	Source
front matter entries	each entry becomes a field (names sanitized via FieldNameSanitizer; block- and flow-style lists become multi-valued fields)
id	front matter id if present, otherwise SHA-256 of the input
title	front matter title, else first level-1 heading
headings	multi-valued field with every heading text (the document outline)
content	plain text body, front matter excluded

The index-data MCP prompt also accepts markdown/md formats.

Maintainer feedback from #69, addressed

Stable id / idempotency: when front matter supplies no id, the document id is derived deterministically from a SHA-256 hash of the input, so re-indexing identical markdown overwrites the same document — matching the tool's idempotentHint. Since LLM-driven conversion of other formats to markdown is non-deterministic, the tool description also instructs clients to supply a stable front matter id when indexing converted content.

Tool steering: the tool description now reads: "Do NOT use for JSON/CSV/XML input; use index-json-documents, index-csv-documents, or index-xml-documents instead. Only convert source content to markdown when there is no dedicated tool for the source format, and supply a stable 'id' in the YAML front matter when doing so."

Testing

• MarkdownIndexingTest — 10 unit tests: front matter extraction, title resolution (front matter wins over H1), heading collection, plain-text body with formatting stripped, field name sanitization, flow- and block-style YAML lists, front matter id, content-hash id stability, empty-input rejection
• Prompt-path tests for markdown and md formats in IndexingServiceTest
• MCP round-trip added to McpClientIntegrationTestBase: index markdown via the tool, then find it by front matter id (runs across all transport × runtime combinations)
• Tool registered in listToolsReturnsExpectedTools and behavior hints asserted in toolsExposeBehaviorHints
• ./gradlew spotlessApply clean; unit tests green locally on JDK 25

Docs updated: README tool table, AGENTS.md format/creator lists.

[adityamparikh]

Looks good! @epugh / @janhoy / @chatman can we merge this please?

[adityamparikh]

Hi @shahzadarain — this PR currently has merge conflicts with main: the README tools section was restructured by #151, and #146 added constructor Javadoc on the same lines this branch touches in IndexingDocumentCreator.

I've resolved the conflicts by merging current main into your branch and opened shahzadarain/solr-mcp#1 against feature/markdown-indexing. If you merge that, this PR will update automatically and become mergeable again. The merge result builds clean (./gradlew build, 328 tests passing, including the MCP stdio integration test that exercises index-markdown-documents end to end).

Of course, feel free to resolve the conflicts your own way instead if you'd prefer different choices — happy to close my PR in that case.

[adityamparikh]

Hi @shahzadarain — friendly follow-up on the merge conflicts here. The resolution PR on your fork, shahzadarain/solr-mcp#1, is still open: merging it into feature/markdown-indexing will make this PR mergeable again. It is still current — only one docs-only commit (#167) has landed on main since it was prepared.

If you would rather resolve the conflicts yourself (e.g. by merging upstream main into your branch locally), that works just as well. Happy to help if anything is unclear!