WEBDEV-8920: Add fetchMetadataField for modeled single-field lookups

[jbuckner]

fetchMetadataValue returns the raw API value, so callers wanting a modeled field build one themselves. The motivating case: collection is a list on an item in several collections but a bare string on an item in one, so reading [0] is wrong half the time.

const result = await metadataService.fetchMetadataField('bra-bfr', 'collection');
result.success?.value;  // 'kodi_archive'
result.success?.values; // ['kodi_archive', 'community']

Takes a field name, not a field class, and derives both the runtime class and the return type from Metadata's own getter. So the field-name-to-field-type mapping isn't duplicated here, and a field added to Metadata works without a change in this package. addeddate comes back as a Date without being told to.

MetadataFieldKey is derived the same way, so only real fields are reachable (identifier and rawMetadata are rejected at compile time). It comes from iaux-item-metadata 1.5.0, where it landed on WEBDEV-8923.

Also rejects a value that isn't a scalar or array of scalars, which would otherwise construct a field whose String() is '[object Object]' inside a successful result.

Heads up for consumers: required member on MetadataServiceInterface, so implementors must add it. Offshoot's MockMetadataService does — that lands with the version bump there.

🤖 Generated with Claude Code

https://claude.ai/code/session_017GxnijHsZmBrHkcuJ5XYf9

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.46154% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.67%. Comparing base (1c72838) to head (00ac134).

Files with missing lines	Patch %	Lines
src/metadata-service.ts	78.46%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   93.14%   90.67%   -2.48%     
==========================================
  Files           4        4              
  Lines         321      386      +65     
  Branches       28       38      +10     
==========================================
+ Hits          299      350      +51     
- Misses         18       32      +14     
  Partials        4        4              

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