Preserve JSON validity when paging list-valued recipe sections

[Trecek]

Problem

get_recipe_section serializes the list-valued errors, warnings, and
ingredients_table sections as JSON and then applies character-offset
chunking. When a section exceeds the delivery bound, a returned content
fragment can split a JSON token or list element and cannot be parsed
independently.

This is residual class 1 from #4314, verified on develop at ac8f653a0
after PR #4330.

Current behavior

• _bounded_recipe_section_response in
  src/autoskillit/server/tools/tools_recipe.py accepts a string and slices it
  at character offsets.
• get_recipe_section passes JSON-serialized strings for errors, warnings,
  and ingredients_table.
• Existing continuation tests cover reconstructed raw/YAML content, not the
  parsing contract for these list-valued sections.
• Current tool and sous-chef guidance does not explicitly define
  concatenate-before-parse behavior.

Required outcome

• Define one explicit, machine-consumable pagination contract for list-valued
  recipe sections.
• Preserve list-element boundaries so every successful page is independently
  valid JSON and retains its original element values and ordering.
• Keep every serialized tool response within the effective delivery byte bound,
  including multibyte data and a single element that cannot fit on an ordinary
  page.
• Return an explicit bounded failure or continuation representation for an
  individually oversized element; do not silently drop it or return an invalid
  fragment.
• Preserve the existing continuation metadata contract (part, total_parts,
  has_more, and next_part) and the behavior of string-valued sections.
• Document the list-page consumption contract in the public tool guidance used
  by agents.

Verification

Add regression coverage for all three list-valued sections:

1. Multiple pages round-trip through JSON parsing and reconstruct the original
   ordered list.
2. Each complete tool response respects the effective byte bound.
3. Multibyte values and a single oversized element have deterministic,
   lossless behavior.
4. Invalid part values fail explicitly.
5. Existing string/YAML continuation behavior remains unchanged.

Provenance and boundaries

• Parent reconciliation: Audit develop's pull/chunk/append paths for bug classes caught in PR #4312 review #4314
• Reference implementation from the closed competing branch: PR Bounded Envelope + Pull Architecture for Delivery-Bound Recipes #4312 commit
  8c00afaa1 (paginate list-typed sections by element, not JSON-string offset). Use it as design evidence, not as a wholesale cherry-pick, because
  PR Rectify: Host-Attested Codex Recipe Delivery and Cumulative-Insertion Immunity #4330 changed the surrounding delivery architecture.
• The absent-backend delivery-limit fallback drift is a separate residual and
  is out of scope for this issue.