fix: Handle dotted package names in script path resolution (#15300)

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## Summary

Fix WindowsRunnable::from_script_path to correctly append extensions
instead of replacing them when resolving executable paths. This resolves
https://github.com/astral-sh/uv/issues/15165#issue-3304086689.

- Add add_extension_to_path helper that appends extensions properly
- Update extension resolution to use the new helper
- Add tests

## Test Plan

Added unit tests for the new and existing functionality that the change
touches. Tested manually locally on Windows.
<!-- How was it tested? -->

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

scripts/packages/package.name.with.dots/README.md

@@ -0,0 +1,6 @@
+# package.name.with.dots
+
+Test package for verifying Windows executable handling with dotted package names.
+
+This package is used to test the fix for the uvx Windows executable bug where package names
+containing dots were incorrectly handled when adding Windows executable extensions.

scripts/packages/package.name.with.dots/pyproject.toml

@@ -0,0 +1,11 @@
+[project]
+name = "package.name.with.dots"
+version = "0.1.0"
+requires-python = ">=3.8"
+
+[tool.uv.build-backend.data]
+scripts = "scripts"
+
+[build-system]
+requires = ["uv_build>=0.8.0,<0.9"]
+build-backend = "uv_build"

scripts/packages/package.name.with.dots/scripts/package.name.with.dots.ps1

@@ -0,0 +1,2 @@
+Write-Host "package.name.with.dots version 0.1.0"
+exit 0

scripts/packages/package.name.with.dots/src/package_name_with_dots/__init__.py

@@ -0,0 +1,3 @@
+"""Test package for verifying Windows executable handling with dotted package names."""
+
+__version__ = "0.1.0"