fix(json): only flag strict ISO dates, not 10-char dashed strings (#1416)

[ousamabenyounes]

Problem (#1416)

rtk aws eks list-clusters (and any AWS list-* op handled by the generic JSON schema path) rendered cluster names like `my-cluster` as `date?` instead of a string. EKS cluster names of length 10 that contain a `-` (very common!) hit the placeholder.

Root cause

`src/cmds/system/json_cmd.rs` extracts a schema for JSON values. The string-classification heuristic treated any string of length 10 containing `-` as a probable date:

```rust
} else if s.contains('-') && s.len() == 10 {
format!("{}date?", indent)
```

That collides with EKS cluster names, slugs, and many short identifiers (`abcd-12-xy`, `foo-bar-ab`, `my-cluster`, …).

Fix

Tightened the heuristic to a strict `^\d{4}-\d{2}-\d{2}$` regex, compiled once via `lazy_static`. Real ISO calendar dates are still classified as `date?`; everything else falls back to `string`.

Test plan

• `cargo fmt --all`
• `cargo test --all` — 1684 passed, 0 failed
• New regression tests in `src/cmds/system/json_cmd.rs`:
  • `test_extract_schema_eks_cluster_name_is_string_not_date` — `{"clusters": ["my-cluster"]}` → `string`, not `date?`
  • `test_extract_schema_iso_date_still_flagged` — `2024-01-15` → `date?` still works
  • `test_extract_schema_dashed_non_date_strings` — `my-cluster`, `abcd-12-xy`, `foo-bar-ab` all → `string`

Closes #1416

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