Improve dictionary null handling in hashing and expand aggregate test coverage for nulls

[kosiew]

Which issue does this PR close?

• Closes Update Fuzz tests to include Dict with null values #16266

Rationale for this change

This change addresses a bug where combine_hashes was applied even if a dictionary value was null, leading to incorrect hash computations.
This was discovered while investigating #16266
Additionally, this PR extends the test coverage for aggregate functions to better validate behavior with dictionary arrays containing nulls.

What changes are included in this PR?

• Fixes logic in hash_dictionary to ensure combine_hashes is only applied when the dictionary value is valid.
• Corrects grammar in error messages for dataset generation expectations.
• Enables null value generation in fuzz tests for dictionary arrays.
• Adds comprehensive tests for aggregate functions (COUNT, SUM, MIN, MAX, MEDIAN, FIRST_VALUE, LAST_VALUE) using dictionary arrays with null keys and values.
• Ensures consistent behavior across single and multi-partition execution.

Are these changes tested?

Yes, extensive new tests are added covering:

• Aggregates on dictionary columns with null keys/values.
• Window functions with null handling (IGNORE/RESPECT NULLS).
• Partitioned vs. unpartitioned execution consistency.

Are there any user-facing changes?

No direct API changes, but query behavior involving dictionary arrays with nulls will now produce correct and consistent results in line with SQL semantics.

[kosiew]

datafusion/common/src/hash_utils.rs | 50 +-
.../tests/fuzz_cases/aggregation_fuzzer/fuzzer.rs | 8 +-
.../tests/fuzz_cases/record_batch_generator.rs | 6 +-

datafusion/core/tests/sql/aggregates.rs | 1442 ++++++++++++++++++++
4 files changed, 1483 insertions(+), 23 deletions(-)

In working on #16266, there was initially a lot of fuzz failures.
I added a lot of tests to reproduce the failures and the bulk of the changes in this PR are the additional tests.