fix support multiple values for python-platform #3440

[asukaminato0721]

Summary

Fixes #3440

PythonPlatform now can represent one platform, multiple platforms, or all.

Platform checks now only fold to true/false when all configured platforms agree; mixed results stay unknown so both branches are type checked.

Test Plan

add test

[Copilot]

Pull request overview

Adds support for specifying multiple python-platform values (and an "all" option) so Pyrefly can avoid incorrectly pruning platform-gated branches when code is intended to run across multiple OS targets.

Changes:

• Update PythonPlatform to represent all or a normalized set of platforms, with serde support for string-or-list config values.
• Extend SysInfo expression evaluation so sys.platform / os.name comparisons can produce “unknown” results when multiple platforms are possible, plus add unit tests for these cases.
• Update user docs and JSON schema to reflect the expanded python-platform configuration surface.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
website/docs/configuration.mdx	Documents python-platform as string-or-list and introduces "all" semantics.
schemas/pyrefly.json	Allows python-platform to be either a string or a non-empty array of strings.
crates/pyrefly_python/src/sys_info.rs	Implements multi-platform/all semantics in PythonPlatform and updates condition evaluation accordingly, with new tests.
crates/pyrefly_config/src/config.rs	Adds config parsing tests for list and "all" values.

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[connernilsen]

Hey Asuka, thanks for making this. I have a few changes, but a few questions remain on some of the implementation details