fix(remote): validate credential fields before use

[poketopa]

Summary

• validate required credential fields before constructing remote options
• return field-specific InvalidArg JavaScript errors for malformed plain JavaScript inputs
• keep credential callback conversion panic-free while preserving valid credential behavior

Problem

Credential fields are optional in the Rust N-API input object so plain JavaScript or TypeScript casts can omit fields that are required by a credential variant. The conversion previously unwrapped privateKeyPath, privateKey, and password, which could panic when libgit2 requested the credential.

Changes

• validate each variant before fetch, push, prune, clone, and submodule operations reach transport setup
• replace unwrap calls with controlled git2 errors as a callback-level safeguard
• preserve the git username fallback and existing libgit2 error conversion
• add local file-transport N-API regression tests and focused Rust unit tests

Validation

• just format
• just lint
• just typecheck
• just test
• git diff --check

All new credential regression cases use local synthetic repositories and values.

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