fix: repair invalid batch graph draft payloads

[SamuelBrudner]

What

• Canonicalize only unambiguous generated create_note aliases (text, content, or body) to raw_content.
• Preserve provider-supplied note titles as metadata.title without weakening the API schema.
• Retry schema-invalid batch output with bounded, sanitized server feedback outside the untrusted source-context boundary.
• Bump the batch prompt contract and add regressions for aliases, conflicting values, title conflicts, and prompt-boundary placement.

Why

The deployed branch fixed production failures where nested provider payloads did not satisfy the strict API request schema, but that fix was based on a much larger mixed release commit. This PR rebuilds the correction directly on current main and contains no onboarding, auth, deployment, frontend, or Beads state.

Safety

Ambiguous/conflicting values remain hard validation failures. Final API validation stays strict, provider feedback is credential-redacted, and human review remains required before applying any operation.

Verification

• 52 focused graph-draft tests passed
• Ruff and git diff --check passed
• Mypy passed across 65 configured source targets
• Full backend run: 4,601 passed / 118 skipped; the four sandboxed wheel-build cases were rerun with a writable populated uv cache and all 9 packaging tests passed