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fix(submissions): resolve form without UUID for permitted collectors (#7216 backport) (#7326)

### 📣 Summary

Backport of #7216 to `release/2.026.27` (was merged directly onto
`release/2.026.23` and never forward-ported to `main`; missing from
`.27`/`.30`/`main` as a result).

Fix a 404 (No XForm matches the given query) when submitting to a form
whose XML does not include the form UUID, for collectors who have
submission permission but are not the form owner.

### 📖 Description

Submissions are matched to their form by UUID first, then by
`id_string`. When the UUID is missing from the submission XML, the
fallback could not find the form for a collector who is not the owner,
so the submission was rejected with a 404.

### 👷 Description for instance maintainers

The `id_string` fallback in `get_xform_from_submission` no longer scopes
the lookup to the submitter's username; authorization is left to
`check_submission_permissions`.

### 💭 Notes

The previous fallback filtered on `user__username=<submitter>` while
`user` is the form **owner**, so it only matched when the submitter was
the owner and returned a 404 otherwise (it never misrouted). Dropping
that filter means a bare `id_string` lookup can now match several forms
when different accounts share the same `id_string`, so we disambiguate
safely:

- resolve by form UUID first (unchanged),
- otherwise a single `id_string` match, no username filter,
- on an `id_string` collision, route to the form whose instance template
the submission conforms to (full field XPaths, common metadata
excluded),
- ownership is used only as a tie-breaker between schema-equivalent
candidates,
- fail closed (404) when no form conforms or the match stays ambiguous.

New `test_get_xform_from_submission.py` covers the path helpers and the
collision routing.

### 👀 Preview steps

1. ℹ️ have two accounts: an owner and a collector
2. owner deploys a form and shares it with the collector, granting "Add
submissions"
3. as the collec... (continued)

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