Fix bytes|str narrowing in HSA2 bridge; add types-requests dev dep

[MrJarnould]

Proposed change

Clears three Pyrefly diagnostics in pyicloud/base.py with no runtime behavior change:

• bad-argument-type — wraps self.session.headers.get("User-Agent", _HEADERS["User-Agent"]) in typing.cast(str, ...) at the TrustedDeviceBridgeBootstrapper.start() call site. requests.Session.headers (a CaseInsensitiveDict) is typed as returning bytes | str, but start() requires str. In practice the header is always a str at runtime; the cast narrows the type without changing behavior.
• untyped-import ×2 — adds types-requests>=2.32.0 to requirements_test.txt so type checkers have stubs for the requests library (no longer warns on requests / requests.models imports).

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New service (thank you!)
• New feature (which adds functionality to an existing service)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Documentation or code sample

Additional information

• This PR fixes or closes issue: N/A
• This PR is related to issue: N/A

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works. (No new test surface — cast is identity at runtime; the existing 2FA / HSA2 bridge tests exercise the call path.)

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📥 Commits

Reviewing files that changed from the base of the PR and between 29315db and b0ecb34.

📒 Files selected for processing (2)

• pyicloud/base.py
• requirements_test.txt

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