Validate large count values before expansion

A resource with count = 9223372036854775807 could reach expansion and trigger Go runtime failures instead of returning a user-facing validation error. The existing checks caught negative and non-whole counts, but did not consistently cap upper-bound values before slice-backed expansion paths.

Introduced a shared maxCount limit, applied it in both count expression evaluation and instance selector compilation, and added validation coverage that rejects values above 2147483647 with a clear diagnostic.

• Turns an infrastructure-as-code crash path into a deterministic validation error before planning work expands resource instances.
• Protects generated OpenTofu configurations where a bad count value could otherwise destabilize validation or apply flows.
• Keeps the count limit consistent across validation and compilation paths instead of relying on platform-dependent runtime behavior.

• Added evaluator and instance selector regression tests for values above the count limit.
• Added a context validation test proving oversized resource counts fail during validation with the expected diagnostic.
• PR validation covered the Go count handling paths changed by the fix and upstream review before merge.