feat: explicit TikZ environment handling in LaTeX backend (#3187)

* feat: explicitly handle tikzpicture as atomic figure nodes

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* fix: resolve copilot review on recursion depth and text labels

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* test: cover tikz fallback label and depth guard

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* fix: address copilot tikz parent and end-tag edge cases

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* feat(latex): store TikZ source in Picture meta code field

Handle tikzpicture environments as real Picture nodes via add_picture and
store raw TikZ in PictureMeta.code (CodeMetaField) with language metadata.

Also update LaTeX backend tests to assert picture meta code behavior and
bump docling-core minimum version to 2.71.0 (includes PictureMeta.code).

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* fix(latex): make TikZ code metadata compatible across docling-core versions

Avoid hard import of CodeMetaField to prevent import-time crashes when older
docling-core is resolved. Use PictureMeta.code when available and retain legacy
CODE fallback otherwise. Update TikZ tests for dual-path behavior and refresh
uv.lock to pin docling-core 2.71.0.

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* refactor(latex): simplify TikZ parent assignment

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* fix(latex): remove legacy PictureMeta guard and use TIKZ code metadata

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* fix: require docling-core 2.73.0 for TIKZ code label

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

* test: remove legacy tikz code-meta compatibility branch

Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>

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Signed-off-by: StealthTensor <rayabharaputrinai@gmail.com>