tests: improve clarity vm tests

[federico-stacks]

Description

Tidy-up and correctness pass over the Clarity VM unit tests (costs.rs, analysis_costs.rs, large_contract.rs, transactions.rs) and the shared test instantiation helpers. The main functional fix addresses a latent mismatch where a test's BurnStateDB epoch and the cost contracts actually deployed in the ClarityDatabase could disagree the tests still passed, but exercised the wrong costs-N contract for the epoch under test.

This issue emerged during PR #6959 where caching for ClarityDatabase epoch was introduced, making the now "fixed" tests failing.

Applicable issues

• fixes #

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[coveralls]

Coverage Report for CI Build 26746080137

Coverage increased (+0.02%) to 85.727%

Details

• Coverage increased (+0.02%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (125 of 126 lines covered, 99.21%).
• 6703 coverage regressions across 124 files.

Uncovered Changes

File	Changed	Covered	%
stackslib/src/chainstate/stacks/db/mod.rs	59	58	98.31%
Total (2 files)	126	125	99.21%

Coverage Regressions

6703 previously-covered lines in 124 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/config/mod.rs	376	68.96%
stackslib/src/net/mod.rs	311	77.84%
stackslib/src/chainstate/stacks/index/storage.rs	284	82.27%
stackslib/src/chainstate/stacks/miner.rs	256	83.23%
clarity/src/vm/database/clarity_db.rs	245	82.18%
stackslib/src/chainstate/stacks/db/transactions.rs	222	97.17%
stackslib/src/net/inv/epoch2x.rs	219	79.55%
stackslib/src/net/chat.rs	202	92.95%
stackslib/src/chainstate/stacks/db/mod.rs	196	86.57%
stackslib/src/core/mempool.rs	193	86.44%

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Coverage Stats

Relevant Lines:	223553
Covered Lines:	191645
Line Coverage:	85.73%
Coverage Strength:	18916153.04 hits per line

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💛 - Coveralls