nostr: add Filter::tag

[mvanhorn]

Description

Adds a Filter::tag(self, tag: Tag) -> Self convenience method (requested in #1359) so callers can add an already-constructed Tag to a Filter directly, instead of manually decomposing it into a SingleLetterTag and a value with the verbose custom_tag(tag.single_letter_tag().unwrap(), tag.content().unwrap()) dance.

The method:

• derives the index key from tag.single_letter_tag(), since only single-letter tags can be indexed in a filter,
• derives the value from the tag's serialized form (Tag::to_vec, the element after the tag name),
• delegates to the existing custom_tag so generic-tag storage stays in one place,
• and is a no-op returning self unchanged when the tag has no single-letter representation or no value.

This is additive and non-breaking.

Closes #1359

Notes to the reviewers

Following @TheAwiteb's guidance on the issue: not all tags fit in a filter, so the method handles the non-fitting case gracefully rather than unwrapping, and reads the value from the tag's serialized form rather than panicking. This gives callers (e.g. building a NIP-66 requirement filter) a clean builder method consistent with the existing event / pubkey / hashtag helpers.

Inline unit tests in crates/nostr/src/filter.rs cover the happy path (a single-letter t tag equals the matching custom_tag / hashtag call), a multi-element single-letter tag indexing the value right after the tag name, chaining .tag() twice to accumulate values, and the two no-op cases (no single-letter representation, and no value). cargo test -p nostr passes (245 + 8 tests).

Checklist

• I followed the contribution guidelines
• I updated the CHANGELOG (if applicable)
• I personally wrote and understood all code in this PR

[TheAwiteb]

utACK

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.64%. Comparing base (83fa932) to head (b4df19b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1383      +/-   ##
==========================================
+ Coverage   75.59%   75.64%   +0.04%     
==========================================
  Files         199      199              
  Lines       32743    32770      +27     
==========================================
+ Hits        24753    24789      +36     
+ Misses       7990     7981       -9     

Flag	Coverage Δ
rust	75.64% <100.00%> (+0.04%)	⬆️

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[TheAwiteb]

@mvanhorn Thank you for your contribution

[yukibtc]

ACK b4df19b