Respect platform guards in conditional expressions

Pyrefly applied platform-condition evaluation at statement level but not inside ternary expressions. Code such as Windows-only subprocess attributes guarded by sys.platform could still produce missing-attribute diagnostics on non-Windows environments.

Added environment-aware boolean evaluation for sys.platform, sys.version_info, os.name, and type-checking guards, then used it in conditional-expression binding and inference so only the reachable branch contributes diagnostics and type information for platform-guarded expressions.

• Removes false positives for real cross-platform Python code that uses ternary expressions around platform-specific stdlib attributes.
• Preserves ordinary constant-condition checking so unreachable branches not tied to configured environment guards still receive diagnostics.
• Keeps generator detection correct when a yield expression appears inside a platform-skipped ternary branch.

• Added Linux and Windows regression coverage for platform-guarded ternary expressions.
• Added skipped-branch yield coverage so generator status remains syntactically correct even when a platform branch is unreachable.
• Landed through Meta codesync with the PR resolution and issue closure pointing to the upstream commit.