Respect platform guards in conditional expressions

[goutamadwant]

Summary

Use the configured Python environment when binding and inferring conditional expressions so only the reachable branch contributes diagnostics and types for sys.platform, sys.version_info, os.name, and type-checking guards.

This preserves the existing behavior for ordinary constant conditions, where unreachable branches are still checked for errors.

Fixes #2558

Test Plan

• cargo test -p pyrefly test_platform_ternary -- --nocapture
• cargo test -p pyrefly test_context_if_expr_unreachable -- --nocapture
• python3 test.py (Scrut tests skipped because Scrut is not installed)

AI disclosure: AI was used to review the code and understanding the codebase.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- ERROR src/prefect/cli/server.py:662:23-58: No attribute `CREATE_NEW_PROCESS_GROUP` in module `subprocess` [missing-attribute]

apprise (https://github.com/caronc/apprise)
- ERROR apprise/url.py:463:17-25: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]
- ERROR apprise/url.py:499:17-25: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]
- ERROR apprise/url.py:504:64-72: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]
- ERROR apprise/url.py:520:17-25: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]
- ERROR apprise/url.py:536:17-25: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]
- ERROR apprise/url.py:543:17-25: Unpacked keyword argument `bool` is not assignable to parameter `string` with type `Buffer | None` in function `_hashlib.openssl_sha256` [bad-argument-type]

trio (https://github.com/python-trio/trio)
- ERROR src/trio/_path.py:133:32-35: Argument `type[PosixPath] | type[WindowsPath] | type[Self@Path]` is not assignable to parameter `cls` with type `type[Self@Path]` in function `pathlib.PurePath.__new__` [bad-argument-type]
+ ERROR src/trio/_path.py:133:32-35: Argument `type[PosixPath] | type[Self@Path]` is not assignable to parameter `cls` with type `type[Self@Path]` in function `pathlib.PurePath.__new__` [bad-argument-type]

AutoSplit (https://github.com/Toufool/AutoSplit)
- ERROR src/utils.py:349:28-36: Could not find name `platform` [unknown-name]

[meta-codesync]

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D113349444.

[stroxler]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@yangdanny97 merged this pull request in 57e7455.