refactor(common): use FirebaseEnv for parameter resolution

[Yusufihsangorgel]

Summary

• route parameter environment lookups through FirebaseEnv
• add unit coverage for mocked environment resolution across parameter types

ListParam and EnumListParam already use this interface. Moving the
remaining parameter classes to the same seam keeps production resolution
unchanged while making their runtime and deployment paths unit-testable with
FirebaseEnv.mockEnvironment.

Fixes #221.

Testing

• dart test test/unit
• dart analyze --fatal-infos lib/src/common/params.dart test/unit/params_test.dart
• dart format --output=none --set-exit-if-changed .

[gemini-code-assist]

Code Review

This pull request refactors parameter resolution in lib/src/common/params.dart to use FirebaseEnv instead of directly accessing Platform.environment. This decouples the environment variable lookup and allows for easier mocking. Corresponding unit tests have been added in test/unit/params_test.dart to verify parameter resolution with mocked environments. I have no feedback to provide as there are no review comments.