fix(forge_config): make global config file optional

[tusharmath]

Summary

Fix a missed case from the config refactor where the global TOML config file was required to be present, causing failures when it doesn't exist.

Context

PR #2685 introduced the forge_config crate as part of a config refactor. During that refactor, the read_global method was added to ConfigReader, but the config file source was registered as required — meaning Forge would fail to start if the global config file (forge.toml) was absent. Since the global config file is optional by design, this was an oversight.

Changes

• Mark the global config file source as required(false) so Forge starts normally even when the file does not exist

Testing

# Ensure no global config file is present and start Forge
rm -f ~/.config/forge/forge.toml  # or wherever the global config lives
cargo run -- --help

# Run crate tests
cargo insta test --accept -p forge_config

Links

• Config refactor PR: feat: add .forge.toml support #2685