[ENG-3355]Fix placeholder consolidation reintroducing stray braces on shared boundary runs

[Deeshath]

Problem

When Word splits placeholders across runs (proofing marks — <w:proofErr>) and fuses one token's closing }} into the same run as the next token's opening {{ — e.g. a run whose text is literally }} in Grade {{ — validate_placeholder_content corrupts the paragraph.

It builds the run/position map once, then edits runs in place per placeholder. When two placeholders share that boundary run, processing the second placeholder rewrites the shared run from the stale cached text, resurrecting the }} the first placeholder's merge had already trimmed. The paragraph's visible text changes ({{a}} becomes {{a}}}}), and after substitution the value renders with a trailing }} (Senior Account Executive}}).

Downstream (Kula ENG-3355): offer-letter merge fields rendered with a stray }} after the value.

Fix

Replace the in-place, static-map editing with a single position-ordered reconstruction pass: walk the joined text once, assign each placeholder wholly to the run holding its opening {{, and assign every other character to its original run. Keeps each placeholder atomic even when tokens share a boundary run, and — unlike the old code — never changes the paragraph's overall visible text.

Tests

spec/docx/paragraph_placeholder_spec.rb builds the exact shared-boundary structure and asserts:

1. consolidation does not alter the visible text (fails on old code: {{offer.job_title}}}});
2. after substitution there is no orphan brace (fails on old code: Senior Account Executive}}).

• New spec: 2 examples. Full suite: 138 examples, 0 failures (136 + 2 new; no regressions).
• Verified against the real customer template: Executive}}, 44,00,000}}, L5}} no longer appear; genuine unresolved placeholders ({{docusign.*}} etc.) left intact.

Note

Please cut a v1.0.4 tag on merge — Kula core pins this fork by tag and will bump to it.