Preserve flow-control windows when rejecting local increments

[adenzhou1350]

WindowManager.window_opened() updates the window before checking its upper bound. A rejected local H2Connection.increment_flow_control_window() therefore leaves the window above the limit even though no WINDOW_UPDATE was emitted. A subsequent valid increment then also fails.

Check the proposed size before assigning it. Add public-API regressions for both connection and stream windows, including a valid update after the rejected one. The limit and exception type are unchanged; this concerns local API rejection, not the separate inbound WINDOW_UPDATE handling.

Validation on Windows CPython 3.12:

• Both new cases fail on the original implementation.
• Full suite: 1,664 passed, with 100% line and branch coverage.
• Changed production file passes Ruff checks; no new test-file lint diagnostics.

The all-version tox matrix and Linux h2spec were not run locally. AI-assisted implementation, independently exercised against the unchanged baseline.