feat(mcp): accept field-fill params on send_documents tool

[Ortes]

Summary

Bring the MCP send_documents tool to parity with the REST POST /submissions endpoint for pre-filling submitter fields. Previously, MCP callers could only set email, name, phone, role per submitter — any values / fields payload was silently dropped.

What changed

lib/mcp/tools/send_documents.rb now accepts per submitter:

• values — pre-fill field values, keyed by field UUID or field name.
• fields — per-field overrides (default_value, value, title, description, readonly, required, validation_pattern, invalid_message).
• readonly_fields — field names to lock for this submitter.
• metadata, external_id, completed, role, message (subject/body).

The input is normalized via Submissions::NormalizeParamUtils.normalize_submissions_params! before handoff to Submissions.create_from_submitters, matching the exact pipeline used by Api::SubmissionsController#create. The rescue block mirrors the same controller so Submitters::NormalizeValues::BaseError and DownloadUtils::UnableToDownload surface as clean MCP error responses.

Backward compatible: existing callers that only pass email / name / phone / role keep working unchanged.

Test plan

• Call send_documents with a values hash keyed by field name → verify values resolve to UUIDs and the submitter is created with the correct defaults.
• Call send_documents with fields: [{ name: "...", default_value: "..." }] → verify the default is merged into values.
• Pass an unknown field name → verify a clean error is returned (UnknownFieldName).
• Pass only email/name (legacy shape) → verify behavior unchanged.

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