fix(cmds/ruby): accept version-only JSON from rubocop/rspec --version

[YOMXXX]

Summary

Fixes #1946. rtk rubocop --version and rtk rspec --version were polluting
stdout with two redundant warnings:

[rtk] rubocop: JSON parse failed (missing field `files` at line 1 column 20)
[rtk] rubocop (JSON parse error): output format not recognized, showing last 5 lines
{"version":"1.86.2"}

This PR detects the minimal {"version":"..."} JSON shape emitted by
--version queries before attempting the full lint/test report parse and
surfaces it as a clean rubocop 1.86.2 / rspec 3.13.2 line.

Root cause

Both modules inject --format json to get structured output. When the user
runs --version, the tool emits {"version":"..."} instead of a full report.
The strongly-typed struct (RubocopOutput { files, summary } /
RspecOutput { examples, summary }) rejects it, the JSON parse error is
logged, and fallback_tail adds a second "output format not recognized"
warning before printing the raw line.

Fix

In filter_rubocop_json and filter_rspec_output, run a lightweight
detector first:

• Parse as serde_json::Value.
• Verify it's an object with a string version key.
• Bail out if any report-specific key is present (files/summary/metadata
  for rubocop, examples/summary/summary_line for rspec) — that means
  it's a real report, not a --version query.
• Otherwise return <tool> <version> cleanly with no warnings.

Real reports that happen to carry a version field (which RSpec's JSON
formatter does) still flow through the existing report parser; covered by
regression tests.

Test plan

• cargo fmt --all
• cargo clippy --all-targets (zero warnings)
• cargo test --bin rtk — 1909 passed, 0 failed
• New tests (8 total, 4 per module):
  • test_filter_rubocop_version_only_json_clean
  • test_filter_rubocop_version_only_json_with_whitespace
  • test_filter_rubocop_version_only_does_not_swallow_real_report
  • test_try_version_only_json_rejects_non_object
  • test_filter_rspec_version_only_json_clean
  • test_filter_rspec_version_only_json_with_whitespace
  • test_filter_rspec_version_only_does_not_swallow_real_report
  • test_try_version_only_json_rspec_rejects_non_object

Files changed

• src/cmds/ruby/rubocop_cmd.rs — add try_version_only_json(), call it before report parse, add 4 tests
• src/cmds/ruby/rspec_cmd.rs — same shape, 4 tests

Fixes #1946