feat: [Geneva Exporter] Add error message parameters to FFI functions for better diagnostics

[lalitb]

Summary

Adds optional error message parameters to all FFI functions to provide rich diagnostic
information for debugging and troubleshooting. Error codes alone are insufficient for
diagnosing issues in production - callers need actual error details (e.g., "Invalid UTF-8
in field 'endpoint'" vs just GENEVA_INVALID_CONFIG).

Changes

Rust (src/lib.rs)

• Added InvalidHandle error code (104) for magic number mismatch
• Added write_error_if_provided() helper for zero-cost error reporting
• Updated validate_handle() to return InvalidHandle instead of InvalidData
• Added (err_msg_out, err_msg_len) parameters to:
  • geneva_client_new()
  • geneva_encode_and_compress_logs()
  • geneva_encode_and_compress_spans()
  • geneva_upload_batch_sync()
• Changed all Err(_e) to Err(e) to capture actual error context
• Changed error formatting from Debug ({:?}) to Display for stability

C Headers

• Updated geneva_errors.h with GENEVA_ERR_INVALID_HANDLE
• Updated geneva_ffi.h function signatures with error message parameters

Examples

• Updated logs_example.c and spans_example.c to demonstrate error handling

Usage

Without error details (zero-cost):

if (geneva_client_new(&cfg, &client, NULL, 0) != GENEVA_SUCCESS) {
    log("Failed to create client");
}

With detailed diagnostics:

  char err_buf[512];
  if (geneva_client_new(&cfg, &client, err_buf, sizeof(err_buf)) != GENEVA_SUCCESS) {
      log("Failed to create client: %s", err_buf);  // e.g., "Invalid UTF-8 in field 'tenant'"
  }

Buffer Handling

• Message shorter than buffer: Full message written, NUL-terminated at actual length
• Message longer than buffer: Truncated to fit, always NUL-terminated (standard C behavior)
• Buffer size = 0 or NULL: No error message written, zero allocation overhead
• All cases: Error code always returned regardless of buffer state

Error messages are always safe and NUL-terminated when buffer provided. Recommended
buffer size: ≥256 bytes for full diagnostics. Thread-safe: no shared state, caller
controls buffer lifetime.

Performance

• Zero heap allocations when error messages not requested (pass NULL)
• Zero heap allocations on success path
• One allocation (immediately freed after copy) when error occurs and caller provides buffer

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 30.00000% with 70 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.0%. Comparing base (a6ce7f5) to head (8db78f3).

Files with missing lines	Patch %	Lines
...try-exporter-geneva/geneva-uploader-ffi/src/lib.rs	30.0%	70 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #474     +/-   ##
=======================================
- Coverage   54.2%   54.0%   -0.2%     
=======================================
  Files         71      71             
  Lines      11436   11501     +65     
=======================================
+ Hits        6203    6220     +17     
- Misses      5233    5281     +48     

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