fix(docx): avoid numbering generated letter paragraphs

From the PR description

Summary

• make generated DOCX section numbering opt-in through an optional numberSections tool argument
• leave letters and other prose documents unnumbered by default
• stop assigning inferred legal numbers to ordinary body paragraphs
• preserve explicit bullets and manually typed numbering
• add DOCX XML regression tests for default and opt-in numbering

Problem

The DOCX generator always configured legal numbering and inferred a numbering level for every paragraph beneath a section heading. This caused ordinary demand-letter prose to render approximately as:

1. DEMAND FOR PAYMENT
1.1 We represent the claimant.
(a) Payment is required within ten days.

A prompt-only fix would be insufficient because the renderer itself applied numbering even when the model did not request it.

Behavior after this change

Documents are unnumbered by default. numberSections: true numbers section headings when explicitly requested or required by a workflow/template, while ordinary prose remains unnumbered. Explicit bullets use Word bullet formatting. Manually typed prefixes such as 1. This reference is intentional. remain literal text when automatic numbering is off.

The dispatcher and renderer both use === true, so missing, null, string, or otherwise malformed values cannot enable numbering accidentally.

Tests

New regression tests inspect word/document.xml (and word/numbering.xml for bullets) inside generated DOCX bytes and cover:

• demand-letter headings and body prose unnumbered by default
• opt-in legal numbering on headings only
• preservation of manually typed numbering when numbering is off
• explicit bullets rendered as bullets rather than legal clauses

Validation completed (rebased onto current main):

• tsc --noEmit in backend - clean
• npm test in backend - 711 passed, 24 skipped
• The four new documentGeneration.test.ts cases all pass

Rebased onto current main

Re-verified the bug has not been addressed upstream in the meantime: documentOps.ts on main still calls legalNumbering(inferredLevel) for inferred paragraphs and still configures the numbering set unconditionally, and numberSections appears nowhere in backend/src.

The rebase was clean. Worth noting explicitly because it touches neighbouring code: this does not conflict with the docx text-value work merged in #328 - that PR's preserves numeric-looking text through an unrelated edit regression test still passes alongside the new numbering tests.

No database, frontend, provider, PDF reconstruction, or product-module changes are included.