Fix nested @catch: stop inner-handled errors from propagating

[christiansany]

Fixes #5339

The bug

When @catch directives are nested, a field error handled by the inner @catch is also counted by the outer @catch, so the outer boundary fires even though, from its perspective, the inner field resolved successfully.

• @catch(to: RESULT): the outer field becomes { ok: false } instead of { ok: true, value: … }.
• @catch(to: NULL): the whole outer object is nulled instead of just the inner field.

It compiles fine and fails silently at runtime. Full write-up and a runnable reproduction are in the linked issue (repro: christiansany/relay-playground#1).

The fix

In RelayReader, whether a @catch boundary "saw" an error was based on _fieldErrors being non-empty — but that list also contains errors already marked handled by an inner @catch (they flow through only for logging). The fix excludes already-handled errors when deciding an outer boundary's outcome:

• _asResult now builds its result from the unhandled errors only ({ ok: true } when none remain).
• The NULL path nulls the field only when there is an unhandled error (_fieldErrors.some(e => !e.handled)).

An inner @catch consumes its error and the outer boundary no longer reacts to it. Errors that are genuinely unhandled at a given level still propagate exactly as before.

Tests

Added regression tests in RelayReader-CatchFields-test.js covering both targets with a server error on the inner field:

• nested @catch(to: RESULT) → outer me is { ok: true, value: { lastName: { ok: false, … } } } (inner catches, outer succeeds).
• nested @catch(to: NULL) → { me: { lastName: null } } (only the inner field nulled).

In both cases the error is still surfaced once in fieldErrors with handled: true.

Testing

• New tests pass; existing RelayReader-CatchFields tests remain green.
• Runtime-only change in RelayReader.js (10 lines); no API or compiler changes.