fix: handle flat alternating header arrays in cache normalizeHeaders

[mcollina]

Description

The cache interceptor's normalizeHeaders function only handled two header
formats:

1. Array-of-pairs: [['key', 'val'], ...]
2. Object: { key: 'val', ... }

However, undici internally uses flat alternating arrays:
['key', 'val', 'key', 'val', ...]. When the cache() interceptor is placed
before dns() in a compose chain (e.g., new Agent().compose(cache(), dns())),
the DNS interceptor converts headers to flat format before passing them to
the cache interceptor, causing normalizeHeaders to throw
'opts.headers is not a valid header map'.

Reproduction

import { Agent, interceptors, request } from "undici";

const { cache, dns } = interceptors;
const cacheThenDns = new Agent().compose(cache(), dns());
const url = "https://www.google.com";
const headers = [];
const r = await request(url, { dispatcher: cacheThenDns, headers });

Fixes #5522

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.67347% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.44%. Comparing base (cb4c2f1) to head (c16c29d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/util/cache.js	83.67%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5536   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         110      110           
  Lines       37328    37418   +90     
=======================================
+ Hits        34881    34966   +85     
- Misses       2447     2452    +5     

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