[COVAL-4475] Treat a null metadata_key as absent in reports validation

[callumreid]

Problem

validate_metadata_key in src/commands/reports.rs decided whether a metadata key was supplied with input.contains_key("metadata_key"). contains_key returns true for an explicit JSON null, but the field deserializes into an Option as None — so a null is effectively absent, and the guard disagreed with what actually got sent.

Two wrong outcomes, both reachable through --input-json on reports create and reports update:

1. --input-json '{"compare_by": "metadata", "metadata_key": null}' passed the client-side check, then sent a metadata comparison with no key for the API to reject. Catching that before the request is the entire point of the guard.
2. --input-json '{"compare_by": "run", "metadata_key": null}' bailed with --metadata-key can only be set when --compare-by is metadata, blocking a valid command over a field that is effectively absent.

Change

The same predicate Greptile's review produced for the sibling validate_custom_dimensions in #102 (commit f55c51e):

let has_metadata_key = input
    .get("metadata_key")
    .is_some_and(|value| !value.is_null());

metadata_key was left out of that PR deliberately — validate_metadata_key predates it on main, and the standing rule is to scope review-bot fixes to new code. This is that fix, on its own.

Note that #102 is still open, so validate_custom_dimensions is not on main yet; these two functions will want to converge on a shared helper once it lands. Left alone here to keep this diff to the defect.

Tests

Two regression tests in tests/cli_tests.rs, mirroring test_reports_create_custom_rejects_null_custom_dimensions and test_reports_create_allows_null_custom_dimensions_without_custom_compare_by from #102:

• test_reports_create_metadata_rejects_null_metadata_key
• test_reports_create_allows_null_metadata_key_without_metadata_compare_by

Verified they fail without the source change (stashed src/commands/reports.rs, re-ran). The failure output is the defect verbatim:

• Case 1 got Error: Authentication failed: Missing API Key — i.e. validation let it through and the CLI really did reach the API.
• Case 2 got Error: --metadata-key can only be set when --compare-by is metadata.

Observed locally on the branch head:

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test --test cli_tests reports — 10 passed, 0 failed

Closes COVAL-4475.

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