fix(interceptors): cache() and deduplicate() silently inert on Client/Pool without opts.origin

[marko1olo]

Problem

When cache() or deduplicate() interceptors are composed onto a Client or Pool directly (not via Agent), opts.origin is not set. Both interceptors use opts.origin as the cache/dedup key without fallback — silently falling back to an empty string, making all requests share a single broken key and effectively disabling the interceptors.

Fix

Added getEffectiveOrigin() helper in lib/util/cache.js that derives origin from opts.origin or falls back to reconstructing it from opts.path. Both cache.js and deduplicate.js use this helper.

Tests

Added test/interceptors/interceptors-on-client.js — 4 tests, all pass.

Split from #5626 as requested by @mcollina.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.50%. Comparing base (2ab178a) to head (534caad).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/interceptor/cache.js	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5628      +/-   ##
==========================================
- Coverage   93.50%   93.50%   -0.01%     
==========================================
  Files         110      110              
  Lines       38455    38467      +12     
==========================================
+ Hits        35959    35969      +10     
- Misses       2496     2498       +2     

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[mcollina]

lgtm