Make sandbox fixtures match what Core sends

[SaladDay]

Makes the Web's sandbox test fixtures match what Core actually sends, ahead of the backend's strict runtime validation of sandbox responses in the shared client (codex/sandbox-client-projection).

Changes

• apps/web/e2e/data/routes.mjs: removed diagnostic: "" from two seeded nodes. Core omits diagnostic while a node is ready (omitempty); it never sends an empty string.
• apps/web/e2e/nodes.spec.ts: the inline deployment mock gains suspension: null (always serialized, null for Docker) and a 64-hex specification_digest, which is present whenever specification is.
• apps/web/src/features/overview/test-fixtures.ts: the node() default no longer sets diagnostic: "".

Verification

• On main: typecheck ✓, web tests 366/366, full web e2e 34/34.
• With the backend's strict client merged in (scratch worktree):
  • agents-client 630/630, web 366/366, full web e2e 34/34.
  • All 47 captured fixture responses pass the strict projection. Before this change, the fixtures above were rejected.

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