Add validateBlobWithPaths for detailed validation diagnostics

Summary:
Extends the SchemaValidator API to optionally track and report the paths to fields that caused validation failures.

The existing `validateBlob` function returns only an enum indicating whether validation passed. This is useful for quick checks but doesn't help diagnose why validation failed. The new `validateBlobWithPaths` function returns a `SchemaValidationResultWithPaths` struct that includes:
- `unknownFieldPaths`: Paths to fields present in the blob but not defined in the schema
- `mismatchedFieldPaths`: Paths to fields where the wire type doesn't match the expected schema type

Path format uses the convention from the `dynamic::Path` class:
- `Person.address.city` for nested fields
- `Person.__unknown_2` for unknown fields (using field ID)
- `Person.items[0]` for list elements
- `Person.tags@"key"` for map keys/values

Implementation details:
- Added `ValidationState` struct to optionally track path state during traversal
- Refactored internal functions to accept optional `ValidationState*` parameter
- Extracted common logic into `validateBlobImpl` to avoid duplication
- Added helper functions `extractTypeName` and `getExpectedTType` for cleaner code

Reviewed By: praihan

Differential Revision: D90638860

fbshipit-source-id: d7b1f89df2fd370aa2e80c7a7f98247747e65477

Author: iahs