Reject an invalid schema every time it's validated

[MohitSAGAR11]

Fixes #133

The meta-validation subscriber marks a schema as validated before checking it, and the mark stayed in place when the check failed. So only the first validate call threw InvalidSchemaError; later calls used the invalid schema, or crashed (TypeError: type.some is not a function for type: 5).

The mark still has to be set before checking, because a meta-schema validates against itself. I tried setting it only after a successful check, and Node aborted with infinite recursion. So it stays where it is, and it's now cleared if validation fails or anything in it throws (including loading or compiling the meta-schema). The schema is then validated again on the next call.

Tests

Added lib/meta-validation.spec.ts with two cases from the issue: validating an invalid schema twice, and validating after a failed compile. Both fail on main and pass with this change.

npm test (15,001 passed), lint and tsc --noEmit are all clean.

Not covered: if two validate calls for the same invalid schema run concurrently (e.g. in Promise.all), the second one still skips meta-validation while the first is in progress, so it can still hit the TypeError. That's the same as before this change. Fixing it would mean storing the in-progress validation and awaiting it instead of a boolean. Happy to do that here or in a follow-up if you want it, but I didn't want to widen the change without asking.

[MohitSAGAR11]

@jdesrosiers
Thanks for the quick review and merge!

Is there anything specific you'd like help with? Otherwise I'll keep doing what I did here: find bugs, open an issue with a repro, and send a fix once you're OK with it.