fix: accumulate remote schema diagnostics instead of overwriting (#116)

[Adityakumar37]

Fixes #116

Problem

compileSchema's main compilation path accumulates schemaErrors / schemaAnnotations
correctly, but addRemoteSchema (called once per entry in options.remotes) overwrote
node.schemaErrors / node.schemaAnnotations on every call instead of appending to them.
It also never set error.data.schemaId, unlike the root compilation path.

As a result, registering multiple remotes only kept the last remote's diagnostics, and any
surviving error had schemaId: undefined — losing provenance and potentially letting invalid
remotes slip past throwOnInvalidSchema.

Fix

In SchemaNode.ts → addRemoteSchema:

• Each error/annotation is now tagged with error.data.schemaId, resolved from the remote's
  own $id (falling back to the resolved url), matching how compileSchema tags root
  schema errors.
• node.schemaErrors / node.schemaAnnotations are now accumulated via .concat(...)
  instead of being reassigned, so registering multiple remotes retains all of their
  diagnostics.

Tests

Added src/tests/issues/issue116.remoteSchema.diagnostics.test.ts:

• Regression test: two invalid remotes → both errors are retained, with correct pointers
  and schemaIds.
• Control test: a single invalid remote still reports correctly (same as before the fix).

Verification

• Reproduced the exact repro script from the issue now returns both errors with correct
  schemaIds instead of just the last one.
• Full test suite passes with no new failures introduced (verified against a clean baseline
  with the fix stashed out).