Restrict pip 25.2 to cflinuxfs3 only to avoid version conflict

This fixes the 'more than one version of pip found' error by ensuring
each stack has exactly one pip version available:

- cflinuxfs3: pip 25.2
- cflinuxfs4: pip 26.1.2
- cflinuxfs5: pip 26.1.2

The buildpack's InstallOnlyVersion() function requires exactly one
version per dependency per stack. This follows the same pattern used
by pipenv which has different versions for different stacks.

Fixes the integration test failure:
TestIntegration/integration/Pip/.../when_using_latest_version/uses_latest_from_manifest

Author: ivanovac

manifest.yml

@@ -77,8 +77,6 @@ dependencies:
   uri: https://buildpacks.cloudfoundry.org/dependencies/pip/pip_25.2_linux_noarch_any-stack_7dc1e988.tgz
   sha256: 7dc1e9882eb18f53150261c3a0cde7d5a39cdb18843c930ee01db8582f6b4f27
   cf_stacks:
-  - cflinuxfs5
-  - cflinuxfs4
   - cflinuxfs3
   source: https://files.pythonhosted.org/packages/20/16/650289cd3f43d5a2fadfd98c68bd1e1e7f2550a1a5326768cddfbcedb2c5/pip-25.2.tar.gz
   source_sha256: 578283f006390f85bb6282dffb876454593d637f5d1be494b5202ce4877e71f2