fix(proxy): avoid registration insert readback

[dengxiayehu]

Problem

A new SIP registration could return 503 Service Unavailable even though the locator row had already been
inserted successfully.

The database-backed locator used ActiveModel::insert(). On MySQL, SeaORM executes the INSERT and then
immediately queries the inserted row by its generated ID to return the complete model. If that follow-up read
does not observe the row yet, SeaORM returns:

RecordNotFound Error: Failed to find inserted item

The registrar treated this as a storage failure and returned SIP 503, although it never used the returned
model.

Reproduction

A regression test reproduces the exact database sequence:

1. The existing-registration lookup returns no row.
2. The INSERT succeeds with one affected row and a generated ID.
3. The immediate read-back returns no row.

Before the fix, the test fails with the same Failed to find inserted item error observed at runtime.

Fix

Use Entity::insert(active_model).exec() for new locator rows.

This checks that the INSERT succeeds without fetching the complete row afterward. Real insert errors are still
propagated; no retry, delay, or error suppression is introduced.

Verification

The regression test was verified with a red-green cycle:

• Before the fix: fails with RecordNotFound Error: Failed to find inserted item.
• After the fix: registration succeeds.