Allow users to insert **/prompt** blocks inside email templates so they can ask,
for example, "Write an introduction for CompanyName working in Industry."

- The **/prompt** command inserts a prompt block inside suitable HTML fields
  (e.g. the body of a `mail.template` or a `mail.compose.message` composer).
- When the email is sent:
  - Dynamic placeholders inside the block are resolved against each record.
  - The resulting string is sent to the LLM service.
  - The block is replaced with the AI response, split on `\n` to preserve paragraphs.
- Supported in all composers (standard, "Share", mass-mailing, etc.);
  multi-record flows results to per-record prompt evaluation.
- Adds an `ai.composer` record to hold the system prompt (e.g. instructing the
  LLM not to add a subject and to treat output as customer-facing).
- Dynamically includes author and the mail's computed language (defaults to
  *en_US*).
- Introduces `prompt_plugin`, registered in `DYNAMIC_PLACEHOLDER_PLUGINS`, with:
  - A read-only AI-prompt icon.
  - Support only for dynamic placeholders inside the block.
- Includes tests in `test_mail_enterprise` for prompt evaluation and multi-record flows.

Task-ID: 4762393