fix: surface proper parsing errors for malformed v2 contracts

[Piskoo]

When a user provided a v2 contract YAML with valid metadata but a structural error in the spec (e.g., materials: ref: vulnerabilities instead of materials: - ref: vulnerabilities), the server returned a misleading "name is required" error. The actual parsing error was silently swallowed in extractMetadata().

The fix uses a lenient proto parse to detect v2 fields (apiVersion and kind) even when the contract has structural errors. When a v2 contract is detected but has parse errors, the actual error is now surfaced to the user.

Before

$ chainloop apply -f testcontract.yml 
WRN API contacted in insecure mode
ERR parsing file testcontract.yml: missing 'metadata.name' field in YAML document of kind "Contract"
exit status 1

After

$ chainlooop apply -f testcontract.yml 
WRN API contacted in insecure mode
ERR invalid contract: error unmarshalling raw message: :8:13 expected sequence, got mapping
   8 |             ref: vulnerabilities
   8 | ............^

exit status 1

[cubic-dev-ai]

No issues found across 2 files

[migmartri]

can you provide an example of the before and after?