Fix unbound prefix recovery when xmlns:dlna is absent

[sfortis]

Summary

The `strict=False` recovery added in #35 anchored its namespace injection on an existing `xmlns:dlna` declaration. Some real-world devices (JBL Authentics, WiiM/LinkPlay) emit `song:*` tags without declaring `xmlns:dlna`, so the injection was silently skipped and the unbound prefix remained, raising `ParseError` when the path was otherwise supposed to recover.

Observable today: `python-didl-lite==1.5.0` is shipped via `async-upnp-client` into Music Assistant, where JBL Authentics 200 owners still see ~100 `unbound prefix` errors per hour and a broken DLNA state-sync loop (downstream tracking: music-assistant/support#4398).

Fix

Anchor the namespace injection on the DIDL-Lite root opening tag (`<DIDL-Lite`) instead of the optional `xmlns:dlna` declaration. The root element is guaranteed to be present in any valid DIDL-Lite document, so recovery works regardless of which namespace declarations the producer chose to include.

Also batches all missing prefix injections into a single regex substitution rather than looping per-prefix.

Test

Added `test_from_xml_string_unbound_prefix_without_dlna_namespace` that exercises the no-`xmlns:dlna` case. The existing `test_from_xml_string_unbound_prefix` (with `xmlns:dlna` present) still passes.

```
$ python3 -m pytest tests/ -v
...
26 passed in 0.18s
```

`ruff check` clean.

Compatibility

• No behavior change when `strict=True` (recovery gated by `if not strict`).
• No behavior change for XML that did contain `xmlns:dlna` and had unbound prefixes (Handle unbound XML prefixes when strict=False #35 case continues to work).
• Recovers an additional class of input that previously raised.

(Replaces #52, which was accidentally opened from the wrong account.)

[StevenLooman]

Thank you for this PR @sfortis. Apologies for not replying earlier, I must have missed the notification mail.

[StevenLooman]

I think a DIDL-Lite XML can use a namespace prefix. E.g., <dlna:DIDL-Lite .... Then this won't work.

Perhaps it should match on <(\w+:)?DIDL-Lite\b.

[sfortis]

That is a real gap, thanks for catching it. I confirmed it against the branch: a document with
a prefixed root fails with unbound prefix exactly as before the fix, because <DIDL-Lite\b
never matches <didl:DIDL-Lite.

One detail came up while applying your suggestion. Changing only the pattern leaves the
replacement as a fixed string, so the prefix is dropped and the opening tag stops matching its
closing tag. The match therefore has to be captured and written back:

xml_string = re.sub(
    r"<((?:[A-Za-z_][\w.-]*:)?DIDL-Lite)\b",
    lambda match: f"<{match.group(1)} {injections}",
    xml_string,
    count=1,
)

I used a callable rather than a string replacement so that the injected declarations are not run
through the backreference expansion of re.sub.

The prefix class is [A-Za-z_][\w.-]* rather than \w+ because a namespace prefix is an NCName,
which allows -, . and _ after the first character.

I ran the candidates over the same inputs to check both points:

root form        | current branch | pattern only    | captured, \w   | captured, NCName
-----------------|----------------|-----------------|----------------|------------------
<DIDL-Lite>      | pass           | pass            | pass           | pass
<didl:DIDL-Lite> | unbound prefix | mismatched tag  | pass           | pass
<my-ns:DIDL-Lite>| unbound prefix | unbound prefix  | unbound prefix | pass

While checking that, I noticed the two detection regexes above have the same limitation, so a
prefix like <my-song:subTitle> is never even reported as missing and recovery does not run at
all, regardless of the root form. Since this PR is about unbound prefix recovery I made all three
consistent:

used_prefixes = set(re.findall(r"<([A-Za-z_][\w.-]*):", xml_string))
defined_prefixes = set(re.findall(r"xmlns:([A-Za-z_][\w.-]*)=", xml_string))

Happy to split that part into its own PR if you would rather keep this one scoped to the anchor.

Two tests are added, one for the prefixed root and one for prefixes containing -, . and _.
Both fail on the previous code and pass now, and the existing suite is unaffected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.82%. Comparing base (bc2d7dc) to head (24beb40).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   94.60%   94.82%   +0.21%     
==========================================
  Files           3        3              
  Lines         408      406       -2     
  Branches       46       45       -1     
==========================================
- Hits          386      385       -1     
  Misses         12       12              
+ Partials       10        9       -1     

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[StevenLooman]

Looks good to me. Can you add a Towncrier fragment, as is required for PRs?