validation: keep script cache enabled when RDTS deployment inactive

[kwsantiago]

In Chainstate::ConnectBlock, when the REDUCED_DATA deployment is inactive at the block being connected, GetBlockScriptFlags does not set REDUCED_DATA_MANDATORY_VERIFY_FLAGS and reduced_data_start_height is set to std::numeric_limits<int>::max().

The per-input loop then evaluates:

if (prevheights[j] < reduced_data_start_height) {   // < INT_MAX, true for every input
    flags_per_input.resize(tx.vin.size(), flags);
    flags_per_input[j] = flags & ~REDUCED_DATA_MANDATORY_VERIFY_FLAGS;   // == flags, bits already absent
}

So flags_per_input gets populated for every input even though each element equals flags. The script-verification results are identical (there is no consensus difference), but CheckInputScripts only writes to the script-execution cache when flags_per_input is empty:

if (cacheFullScriptStore && (!pvChecks) && flags_per_input.empty()) {
    validation_cache.m_script_execution_cache.insert(hashCacheEntry);
}

The net effect is that the script-execution cache is silently bypassed for every block connected while REDUCED_DATA is inactive, i.e. the entire pre-activation history on every node syncing from genesis. This is a pure performance regression with no effect on validation results.

This gates the per-input branch on reduced_data_active so flags_per_input stays empty when the deployment is inactive, restoring the cache. The DeploymentActiveAt result is also computed once instead of twice.

Behavior when REDUCED_DATA is active is unchanged: inputs spending pre-activation coins still receive per-input flags (and correctly forgo caching for those transactions).

Fix suggested by @chrisguida in review of #238.

Verification: builds cleanly; feature_rdts.py passes.

[pdath]

ctACK ff900da

The complexity of this change exceeds my knowledge, so I can not do a human code review.
I reviewed the code with Code Rabbit and found no issues.

I tested using the following commands on Ubuntu 24.04 running on ARMv8-A (64-bit):

git clone https://github.com/bitcoinknots/bitcoin.git pr323
cd pr323
git fetch origin pull/323/head:pr323
git switch pr323

cmake -B build
-DRDTS_CONSENT=IMPLICIT
-DBUILD_GUI=OFF
-DCMAKE_BUILD_TYPE=Release
nice cmake --build build -j4

cd build
ctest -R "validation_tests|mempool_tests" --output-on-failure
python3 test/functional/test_runner.py mempool_reorg.py feature_rdts.py feature_reduced_data_utxo_height.py

[kwsantiago]

@chrisguida mind taking a look? Four-line fix, keeps the script cache enabled while the RDTS deployment is inactive.