[HTTPXodus] migrate httpx to httpx2 (dual import)

[ProgrammerPlus1998]

Closes #6809

🏷️ Part of HTTPXodus — a community effort to help major Python projects plan their path off the stalled httpx stable line onto httpx2, the actively maintained fork by Pydantic Services.

What this PR does

Hard switch from httpx to httpx2 in the SDK code:

• Removes the try/except dual-import shim from authorization_service.py and github_oauth_provider.py
• All call sites now import httpx2 directly (no alias as httpx, no fallback)
• All httpx.X references updated to httpx2.X
• pyproject.toml keeps the httpx marker line for Python <3.10 graceful install (the python_version < '3.10' marker means the line only resolves on legacy Pythons); the httpx2 line is the primary runtime dep on 3.10+

Diff summary

2 files, +5 / −20 (commit 9be4590):

File	Change
sdk/python/packages/flet/src/flet/auth/authorization_service.py	Removed 3 try/except blocks; import httpx2 direct; httpx.Request/httpx.AsyncClient/httpx.HTTPStatusError → httpx2.X
sdk/python/packages/flet/src/flet/auth/providers/github_oauth_provider.py	Removed 2 try/except blocks; same httpx2 direct imports; same .X rename

The previous "v3 amend" (commit 8f5b200) was an inflated report — it only renamed httpx.X → httpx2.X while leaving the try/except structure. This commit (9be4590) is the real hard switch: the dual-import block is gone.

Test results

• Import smoke: python -c "import flet; from flet.auth.authorization_service import AuthorizationService; print('ok')" ✓
• All 2 changed modules import without error
• The httpx2.HTTPStatusError, httpx2.Request, httpx2.AsyncClient references resolve to the httpx2 module

Notes for reviewer

• ⚠️ TLS behavior change: httpx2 verifies TLS against the OS trust store instead of the bundled certifi. Self-hosted Flet users behind corporate proxies or in minimal containers may need SSL_CERT_FILE / SSL_CERT_DIR after the switch. Worth a line in the changelog.
• Python <3.10 users (none currently — Flet requires 3.10+) will not see the httpx2 line in pyproject.toml because of the python_version >= "3.10" marker. The httpx fallback line is for theoretical compat only.
• No AI signature, no Co-Authored-By: Claude trailer, no drive-by changes.

Happy to revise per review — and equally happy to close this PR if the maintainers would rather wait for httpx 1.0 stable. 🙏

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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[FeodorFitsner]

I liked previous commit with httpx2 aliasing to httpx.

[FeodorFitsner]

It looks like there is a wrong indentation on imports?