Add field path details to the conformance runner output

[timostamm]

For the following conformance failure, the runner shows exactly the same information for expected and actual result:

--- FAIL: standard_constraints/map (failed: 1, skipped: 0, passed: 0, total: 1)
    --- FAIL: recursive/invalid
       input: [type.googleapis.com/buf.validate.conformance.cases.MapRecursive]:{val:{key:1 value:{}}}
        want: 1 validation error(s)
          1. val[1].val: string.min_len (string.min_len)
             value length must be at least 3 characters
         got: 1 validation error(s)
          1. val[1].val: string.min_len (string.min_len)
             value length must be at least 3 characters

The cause for the failure is Violation.field: The path has an element with a int_key instead of the expected uint_key, which doesn't show in the string representation of the path.

This PR changes the output for a validation error result to include the Protobuf representation of field paths:

--- FAIL: standard_constraints/map (failed: 1, skipped: 0, passed: 0, total: 1)
    --- FAIL: recursive/invalid
       input: [type.googleapis.com/buf.validate.conformance.cases.MapRecursive]:{val:{key:1  value:{}}}
        want: validation error (1 violation)
          1. constraint_id: "string.min_len"
             message: "value length must be at least 3 characters"
             field: "val[1].val" elements:{field_number:1  field_name:"val"  field_type:TYPE_MESSAGE  key_type:TYPE_UINT32  value_type:TYPE_MESSAGE  uint_key:1}  elements:{field_number:1  field_name:"val"  field_type:TYPE_STRING}
             rule: "string.min_len" elements:{field_number:14  field_name:"string"  field_type:TYPE_MESSAGE}  elements:{field_number:2  field_name:"min_len"  field_type:TYPE_UINT64}
         got: validation error (1 violation)
          1. constraint_id: "string.min_len"
             message: "value length must be at least 3 characters"
             field: "val[1].val" elements:{field_number:1  field_name:"val"  field_type:TYPE_MESSAGE  key_type:TYPE_UINT32  value_type:TYPE_MESSAGE  int_key:1}  elements:{field_number:1  field_name:"val"  field_type:TYPE_STRING}
             rule: "string.min_len" elements:{field_number:14  field_name:"string"  field_type:TYPE_MESSAGE}  elements:{field_number:2  field_name:"min_len"  field_type:TYPE_UINT64}

• The headline changes from "1 validation error(s)" to "validation error (1 violation)" to match API types
• All values are labeled for clarity
• Strings are shown as string literals for clarity
• field and rule show the string representation and the prototext representation

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 11, 2025, 3:15 PM