test: add edge-case tests for fct_shuffle()

[LeonidasZhak]

Add edge-case tests for fct_shuffle() covering common data cleaning scenarios. Currently fct_shuffle() has only 1 test (basic reproducibility check).

Tests added

1. NA values preserved: Verifies NA positions are maintained after level shuffling
2. Empty factor: Verifies empty input returns empty factor with levels intact
3. Single level factor: Verifies single-level factor is unchanged
4. Unused levels preserved: Verifies unused levels remain after shuffling
5. Ordered class preserved: Verifies ordered factor class is maintained

Validation

• New tests: [ FAIL 0 | WARN 0 | SKIP 0 | PASS 10 ]
• Full test suite: [ FAIL 1 | WARN 0 | SKIP 0 | PASS 228 ] (1 pre-existing snapshot mismatch in test-collapse.R due to lifecycle wording change, unrelated)
• Consistent with edge-case test pattern used for fct_rev() (PR test: add edge-case tests for fct_rev() #399)

[LeonidasZhak]

Withdrawing this small automated PR while I consolidate an oversized batch of contributions and reduce maintainer review burden. Sorry for the noise, and thank you for maintaining the project.