Adopt tidy-maven-plugin for consistent POM formatting

## Background

Following the successful Spotless adoption for Java source formatting (#4408), the POM files themselves remain inconsistently formatted. Running `tidy:pom` reveals structural inconsistencies across all 9 POM files in the project.

## Current State (tidy:pom analysis)

| File | Moved Elements |
|------|---------------|
| pom.xml (root) | 186 |
| javaparser-symbol-solver-testing/pom.xml | 74 |
| javaparser-core-testing/pom.xml | 47 |
| javaparser-core-testing-bdd/pom.xml | 47 |
| javaparser-core-serialization/pom.xml | 39 |
| javaparser-symbol-solver-core/pom.xml | 28 |
| javaparser-core/pom.xml | 13 |
| javaparser-core-generators/pom.xml | 10 |
| javaparser-core-metamodel-generator/pom.xml | 10 |

**Summary:** 9 files, 245 insertions / 238 deletions, 56 mixed tab/space indentation fixes, 1 schema URL update (http→https).

## Issues Found

1. **Element ordering inconsistency** — `<modules>`, `<properties>`, `<dependencies>` placed in non-canonical order across modules
2. **Mixed indentation** — 56 lines use tabs where spaces are the project standard
3. **Schema URL** — Uses deprecated `http://` instead of `https://` for Maven XSD

## Proposal

Follow the same pattern established in #4408 for Spotless adoption:

1. **Add `tidy-maven-plugin` to pluginManagement** (configuration only, no enforcement yet)
2. **Run `tidy:pom` once** to reformat all POMs to canonical Maven ordering
3. **Add the reformat commit to `.git-blame-ignore-revs`** (infrastructure already exists)
4. **Add `tidy:check` to CI** (parallel to Spotless check — ensures POMs stay tidy)

## Benefits

- **Consistency** — Developers always know where to find `<properties>`, `<dependencies>`, etc.
- **Reduced merge conflicts** — Canonical ordering eliminates "where do I put this?" disagreements
- **CI-enforceable** — Same philosophy as Spotless: format once, enforce forever
- **Low risk** — Zero functional impact, purely cosmetic

## Considerations

- One-time git blame noise (mitigated by `.git-blame-ignore-revs`)
- Open PRs touching pom.xml would need rebasing (currently low impact)

## References

- [tidy-maven-plugin](https://www.mojohaus.org/tidy-maven-plugin/)
- #4408 — Spotless formatting adoption (same approach, successfully implemented)
- `.git-blame-ignore-revs` already in place from Spotless reformat

Happy to implement this if the approach is acceptable.

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