#36066 validation: Separate check-only version of ConnectBlock

• Clarify mutability guarantees when checking blocks without applying state updates
• Eliminate boolean check flags and mutable dummy objects in validation tests

Separates the check-only execution path in `Chainstate::ConnectBlock` into a dedicated `TestConnectBlock` method taking `const CBlockIndex&`. The shared UTXO validation checks are moved into an internal helper `ConnectBlockChecks`, eliminating the public `fJustCheck` boolean parameter.

Problem: In check-only validation mode (used by `TestBlockValidity` and fuzz testing), `ConnectBlock` required a mutable `CBlockIndex*` even though the block index was not modified, obscuring mutability guarantees and requiring caller-side dummy objects.

P3 · cleanup

• P3 because it is a low-risk internal code cleanup with no performance or security impact
• Improves const correctness for developers and aligns with work toward stateless validation

The PR is a code-hygiene refactor that improves const-correctness in validation interfaces and removes a boolean flag (`fJustCheck`). As noted in the description, 'The mutability of the pindex parameter is in sync with the semantics' and it aligns with efforts to decouple validation side effects (#35904).

Membership: Directly alters `Chainstate::ConnectBlock` and introduces `TestConnectBlock` in src/validation.cpp and src/validation.h.

Factors: security/stability 0, bug 0, performance 0, user value 0, leverage 1

• Ready for review
• Latest update addressed review feedback and resolved a silent fuzz test conflict

The author resolved previous review suggestions from l0rinc and fixed a silent merge conflict with fuzz tests in the latest push.

Author status: active

Resolved concerns:

• l0rinc suggested passing the block index by const reference, reusing the stored block hash, and updating related comments, all of which the author applied.
• purpleKarrot noted that `Chainstate` still mutates internal timing and counter state during check-only calls; the author acknowledged this as part of longer-term work toward stateless validation in #35904.

• Concept approval because separating the check-only path clarifies mutability (l0rinc)
• Noted that internal chainstate mutations remain until later stateless validation work (purpleKarrot)

Concept ACK from l0rinc; specific review requests were addressed in the latest push.

l0rinc provided a Concept ACK with change requests regarding const references and comments, which the author incorporated. purpleKarrot raised architectural considerations around side effects without blocking.

• l0rinc: 'Concept ACK... Separating the check-only entry point makes the CBlockIndex mutability clearer and removes fJustCheck from the public ConnectBlock() interface.'
• purpleKarrot: 'What about the mutability and mutations of the Chainstate... Please have a look at #35904'
• optout21: 'Applied review suggestions, minor changes.'

Review verdicts (DrahtBot): 0

• Concept ACK: l0rinc

11 lines under test/bench/ci.

• src/validation.cpp +44/-13
• src/validation.h +16/-1
• src/test/fuzz/connect_block.cpp +4/-5
• test/functional/interface_usdt_utxocache.py +1/-1

This PR refactors `Chainstate::ConnectBlock` by separating its check-only mode into a distinct `TestConnectBlock` method that accepts a `const CBlockIndex&`. This removes the `fJustCheck` boolean parameter and clarifies const-correctness for callers that only validate blocks without mutating chainstate, such as `TestBlockValidity` and fuzz tests. The work is code hygiene that also supports ongoing modularization toward side-effect-free validation (#35904). Review feedback from l0rinc has been incorporated and the PR is ready for review.