chore: resolve ty diagnostics across runtime and nodes

[WH-2099]

Summary

• resolve the remaining ty diagnostics across graph runtime, model runtime, nodes, variables, and the associated tests
• split the cleanup into low-risk/protocol-only fixes first, then handle the remaining path-sensitive diagnostics with explicit narrowing and constructors
• add a regression test for the intentional array[string] validation error path

Why

uv run ty check was failing on a mix of protocol mismatches, over-narrow annotations, and a handful of code paths where the checker could not prove the existing invariants. This branch makes those invariants explicit without broad runtime rewrites.

Impact

• ty check is green again
• make pre stays green after the cleanup
• invalid array[string] outputs now raise a deliberate OutputValidationError instead of surfacing a raw TypeError
• the second batch of fixes was reviewed separately for logic-change risk; no material regression risk was identified

Validation

• make pre
• env UV_CACHE_DIR=/tmp/uv-cache uv run ty check

Closes #22

[laipz8200]

It's almost LGTM. Please consider my comments.