fix(internal): shared mutable default arguments and type identity comparisons

[harshadkhetpal]

Summary

Small correctness fixes in ddtrace/internal/ (ruff B006/E721), no behavior change:

1. http.py — request(..., headers={}): mutable shared default on a hot path. Changed to headers=None with _headers = dict(headers or {}) (was already copied, so semantics identical).
2. remoteconfig/worker.py — register_callback(..., capabilities=[]) → = (): the annotation is Iterable, so an immutable empty tuple satisfies it with zero call-site changes and removes the shared-list hazard.
3. gitmetadata.py — two type(x) != str → is not str (identity is the intended semantics for type objects).
4. encoding.py — type(err) == bool → is bool (same intent, E721-clean; deliberately not isinstance to preserve the exact-type check that excludes int).

Checklist notes

Lint-only/internal hygiene — no user-facing behavior change, so presumably no-changelog; happy to add a release note if maintainers prefer. python -m py_compile and ruff check --select B006,E721 pass on all four files.

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[chatgpt-codex-connector]

Update the documented callback default

Changing this introspectable default to () leaves the RemoteConfigPoller API guide at ddtrace/internal/README.md:140 advertising register_callback(product, callback, capabilities=[]). Update that signature so maintainers following the internal guide see the actual contract.

AGENTS.md reference: AGENTS.md:L17-L17

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