fix(docs): make structured edits revision-safe

[hentt30]

Problem

docs.replaceText renders a tab to plain text, then uses offsets in that rendered string as Google Docs API indexes. Those coordinate systems diverge around rich links/person chips, tables, and other structural elements, so a replacement can delete adjacent content. It also writes without requiredRevisionId, allowing a concurrent edit to shift ranges between read and write.

Change

• resolve literal matches from real paragraph-element startIndex / endIndex
• apply multiple replacements from highest index to lowest
• lock writes with writeControl.requiredRevisionId
• return explicit match counts, including no-op
• add structure/preview/section-replacement tools with unique heading anchors, expected text, and revision checks
• document safe section-edit flow

Verification

• npm test: 630 passed, 1 skipped
• npm run lint
• npm run format:check
• npm run build
• live disposable Google Doc integration: rich link, table, two tabs, styled headings; section replacement preserved content outside the selected range

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