refactor(token-tracker-http): decompose 238-line trackTokenUsage into testable top-level functions (#4937)

* Initial plan

* refactor: decompose trackTokenUsage into testable sub-functions

Extract the two inner functions from the 238-line trackTokenUsage closure
into top-level functions that accept explicit state:

- initHttpState({ streaming, compressed, contentType, contentEncoding })
  Initialises the mutable tracking state object so it can be constructed
  independently in tests.

- createChunkHandler(state, { requestId, provider }) → handleDecodedChunk
  Returns the decoded-chunk handler; mutates `state` via explicit reference
  instead of closing over outer-scope variables.

- wireListeners(proxyRes, decompressor, state, onChunk, onFinalize)
  Encapsulates event-listener wiring (decompressed and uncompressed paths).

- finalizeHttpTracking(state, proxyRes, opts)
  The former inner `finalizeTracking` closure, now taking all inputs as
  explicit parameters so it can be unit-tested with a synthetic state object
  and a minimal `{ statusCode }` proxyRes mock.

trackTokenUsage is now ~30 lines (init → create handler → wire → finalize).
The public module API is unchanged; token-tracker.js requires no updates.

New exports: createChunkHandler, finalizeHttpTracking (alongside trackTokenUsage).
New test file: token-tracker-http.unit.test.js — 18 unit tests covering both
sub-functions directly without constructing a full HTTP response stream.

All 1062 existing tests continue to pass.

Closes #4934

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot