Report malformed schema JSON as a compile error instead of panicking

[danieleades]

Problem

import_types! unwraps the result of serde_json::from_reader, so a JSON syntax error in the schema file (stray comma, truncated file, ...) panics the proc macro. Users see an opaque error with no indication of what went wrong or where:

error: proc macro panicked
 --> src/main.rs:1:1
  |
1 | typify::import_types!(schema = "bad.json");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: message: called `Result::unwrap()` on an `Err` value: Error("key must be a string", line: 4, column: 34)

Fix

Map the parse error to a spanned syn::Error, matching the existing handling of file-open failures. serde_json's line/column information is preserved in the message:

error: couldn't parse file bad.json: key must be a string at line 4 column 34
 --> src/main.rs:1:32
  |
1 | typify::import_types!(schema = "bad.json");
  |                                ^^^^^^^^^^

This addresses the malformed-JSON half of the diagnostics concerns raised in #516.

Trade-offs

None.

Testing

• cargo test --workspace --locked passes.
• The repo's trybuild usage only covers pass cases (no compile_fail harness), so the before/after output above was verified manually with a scratch crate depending on typify by path with a malformed bad.json.