Fix escaped Jinja delimiters split across Word runs

[CAOShurong]

Closes #548.

Problem

Jinja's escaped delimiters ({_{, }_}, {_%, %_}) render correctly when all three characters are in one Word run. When Word splits those characters across separate <w:r> elements, the run markup remains between them and the final delimiter replacement never sees a contiguous token.

Fix

• Join XML-tag-only gaps inside the four exact escaped delimiter tokens before the existing Jinja cleanup.
• Refuse to cross a paragraph boundary, so unrelated text and structure are not removed.
• Add a generated DOCX regression covering both single-run and split-run delimiters.
• Preserve the owner's prior false-positive examples ({a_{hello and {f_{) unchanged.

Verification

• Reproduced the bug from the exact DOCX attached to Escaping delimiters cannot render properly #548: current master fixed the four single-run examples but left all four split-run examples escaped.
• With this change, all eight examples render to {%, %}, {{, and }} as intended.
• The new regression fails on current master and passes on this branch.
• Python 3.9 and 3.13: 37 non-CLI test scripts passed; the module CLI was also run separately and generated a valid DOCX.
• flake8 . --count --max-line-length=127 --show-source --statistics: 0 findings.
• Clean sdist/wheel build and clean wheel installation passed; the installed wheel reproduced the corrected behavior.

OpenAI Codex assisted with reproduction, implementation, and test execution. I reviewed the diff and all reported results before submitting.