[FIX] purchase_request: render HTML notification body correctly

[les-adhoc]

Problem

When a purchase order created from a purchase request is confirmed, the
notification posted on the request showed its HTML tags (<h3>, <ul>,
<li>) as literal text instead of rendered markup. Reproducible on a
real v19 database; the same broken email reached the user who approved the
request.

Root cause

PurchaseOrder._purchase_request_confirm_message_content built the body as a
plain str and passed html_escape(line["name"]) — which returns a
Markup — as an argument to self.env._().

In Odoo 19, Environment._ escapes the entire translated string whenever
any of its arguments is a Markup:

# odoo/tools/translate.py
if any(isinstance(a, Markup) for a in args.values()):
    translation = escape(translation)

So the surrounding <h3>/<ul>/<li> markup got escaped together with the
value, and rendered as text.

PurchaseRequestAllocation._purchase_request_confirm_done_message_content
had the exact same pattern (HTML in the msgid + html_escape() argument to
self.env._()) and is fixed here too.

Fix

Build the body with Markup(...).format(...) from the start, keep the
translatable strings free of HTML, and apply html_escape() outside of
self.env._() — mirroring the existing
_purchase_request_confirm_done_message_content method which already does it
correctly. The redundant Markup(message) wrap at the call sites is dropped
since the builders now return Markup.

Translatable terms (.pot) updated accordingly: the HTML is no longer part
of the msgids.

Notes

• No functional/data change; output is identical HTML, now rendered instead
  of escaped.
• Version bumped to 19.0.1.0.2.