config: Return default if sub-config key doesn't exist

[shivaraj-bh]

resolves #353

OmConfigError::UnexpectedAttribute which was moved from QualifiedAttrError::UnexpectedAttribute of the now deleted qualified_attr.rs module seems unnecessary? We could just return the default if the sub-config key doesn’t exist?

The error UnxpectedAttribute is also misleading because we make the user think that we expected a correct one from them, but in reality it is the mistake of whoever is calling OmConfig::get_sub_config_under with incorrect root_key (see the function signature for reference).

[srid]

I found one more bug:

unexpected fragment 'default.omnix' in flake reference

Previous behaviour can be reproduced using (which runs fine):

nix shell nixpkgs#nixVersions.nix_2_18 -c nix --accept-flake-config run github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973 -- ci run github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix

New behaviour using this PR:

❯ nix shell nixpkgs#nixVersions.nix_2_18 -c nix --accept-flake-config run github:juspay/omnix/omconfig-regression -- ci run github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix

👟 Reading om.ci config from flake
❄️  nix --accept-flake-config --version️
❄️  nix --extra-experimental-features nix-command --accept-flake-config show-config --json️
❄️  nix --accept-flake-config flake metadata --json 'github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix'️
Error: Nix command error: Command error: Process exited unsuccessfully. exit_code=Some(1) stderr=error: unexpected fragment 'default.omnix' in flake reference 'github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix'


Caused by:
    0: Command error: Process exited unsuccessfully. exit_code=Some(1) stderr=error: unexpected fragment 'default.omnix' in flake reference 'github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix'

    1: Process exited unsuccessfully. exit_code=Some(1) stderr=error: unexpected fragment 'default.omnix' in flake reference 'github:juspay/omnix/0ed2a389d6b4c8eb78caed778e20e872d2a59973#default.omnix'

[srid]

Given the brittle nature of these code paths, could you add tests covering various cases of ci run <ARG>?

https://github.com/juspay/omnix/blob/main/crates/omnix-cli/tests/command/ci.rs

At least, we need cases covering:

• Config regression? Error: Unexpected attribute #353
• config: Return default if sub-config key doesn't exist #354 (comment)