fix(dispatcher): normalize headers before composed interceptors

[GiHoon1123]

Fixes #4336.

When a dispatcher is composed with interceptors, request headers can reach them as an object, a flat array, a Headers instance, or another iterable. Normalize them once when the composed dispatcher is called so interceptors see the same shape regardless of how the request was created.

Already-normalized plain objects are passed through unchanged. Other header forms are converted to a lower-case object, with repeated names kept as arrays. The original request options are not modified.

Added coverage for object, flat-array, Headers, and custom iterable inputs, including the DNS redispatch path.

Tests:

• borp --timeout 180000 test/interceptors/interceptors-on-client.js test/interceptors/cache-query-params.js test/interceptors/redirect.js
• node --test --test-name-pattern '#5522|compose\\(\\) normalizes' test/interceptors/dns.js test/interceptors/interceptors-on-client.js
• eslint lib/dispatcher/dispatcher.js test/interceptors/interceptors-on-client.js

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.46154% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.42%. Comparing base (b73952a) to head (88e5463).

Files with missing lines	Patch %	Lines
lib/dispatcher/dispatcher.js	88.46%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5780      +/-   ##
==========================================
- Coverage   93.51%   93.42%   -0.10%     
==========================================
  Files         110      110              
  Lines       39359    39435      +76     
==========================================
+ Hits        36808    36843      +35     
- Misses       2551     2592      +41     

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[mcollina]

I'm not convinced by this change, as it's significantly perf cost.

Wdyt @ronag?

[ronag]

Yea. This is probably not a good idea...

[GiHoon1123]

That makes sense. The current implementation wraps every interceptor boundary, so it does more work than necessary for each request.

I’m thinking of changing it to normalize once at the composed dispatcher entry, skip plain objects that are already normalized, and keep the resulting object through interceptor redispatches. Would that address the performance concern, or would you prefer a different approach?

[GiHoon1123]

I reworked this to avoid wrapping each interceptor boundary. Headers are now normalized once when the composed dispatcher is entered, and already-normalized plain objects are passed through without a copy. The existing DNS redispatch cases still pass with the normalized object.

The updated commit is 88e54638. The relevant interceptor tests and lint pass locally.