[Censys enrichment] Split converter into package, fix manifest logo path

[alimarhoon-censys]

Proposed changes

• Update the logo field in __metadata__/connector_manifest.json to point at internal-enrichment/censys-enrichment/__metadata__/logo.png. The image was already in that location; the manifest was pointing one directory over.
• Reshape src/censys_enrichment/converter.py into a converters/ package (base.py, host.py, certificate.py, domain.py) plus a shared builder.py. Same layout as some other connectors. No behavior change - the 23 existing tests all pass; the only test-file changes are import-path updates.
• Collect the typed exceptions (EntityNotInScopeError, MaxTlpError, EntityTypeNotSupportedError, EntityHasNoUsableHashError) into a new errors.py. They were previously co-located with the code that raised them.
• Add a @field_validator("scope") in settings.py so misconfigured CONNECTOR_SCOPE values are surfaced at startup rather than at dispatch time. Mirrors the pattern used in other connectors.
• Add a None-check in CensysStixBuilder.add_services for the case where add_certificate returns None, so the relationship creation is skipped cleanly.

Related issues

• Closes [Censys enrichment] Follow-up work: manifest logo path + converter restructure #6287

Checklist

• I consider the submitted work as finished
• I have signed my commits using GPG key.
• I tested the code for its functionality using different use cases
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

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