Enable Dijkstra-era transaction construction and balancing

[palas]

Context

Dijkstra transactions can now be built, fee-estimated and auto-balanced.

The experimental makeUnsignedTx, estimateBalancedTxBody and makeTransactionBodyAutoBalance all work in Dijkstra.

Extra key witnesses become key-hash guards, the era's replacement for required signer hashes.

Breaking: BalanceIsNegative now carries the era's UnsignedTx instead of a Conway-specific one.

No new dependencies: everything builds against released packages.

How to trust this PR

Minimal generalisation changes. Use Hide whitespace when viewing the diff.

The one genuine API change is BalanceIsNegative carrying UnsignedTx era (a strict generalization of the previously Conway-hardcoded field).

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Pull request overview

This PR extends the experimental transaction-construction/balancing pipeline to work in the Dijkstra era, primarily by removing Dijkstra “TODO/unsupported” stubs and reusing the existing Conway-era logic with the necessary era-specific pivots. It also introduces a (documented) breaking change by generalizing BalanceIsNegative to carry an era-generic UnsignedTx.

Changes:

• Enable Dijkstra support in transaction construction (makeUnsignedTx) by translating extra key witnesses into key-hash guards (Dijkstra’s replacement for required signer hashes).
• Enable Dijkstra support in experimental fee estimation and auto-balancing by adding Dijkstra-era branches mirroring the Conway-era logic.
• Generalize BalanceIsNegative to carry UnsignedTx era (breaking), and add a changelog fragment marking feature + breaking.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cardano-api/src/Cardano/Api/Tx/Internal/Fee.hs	Switch Conway-onwards constraints use-site to obtainCommonConstraints to support Conway+ (incl. Dijkstra) proposal-procedure rebuilding.
cardano-api/src/Cardano/Api/Tx/Internal/Body.hs	Remove Dijkstra “unsupported” stub for vote witness extraction by using the Conway-onwards constraints path for all Conway+ eras.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/Fee.hs	Add Dijkstra-era implementations for fee estimation and auto-balancing; generalize BalanceIsNegative to carry UnsignedTx era.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/BodyContent/New.hs	Make makeUnsignedTx era-generic; translate extra key witnesses into Dijkstra guards; enable vote witness extraction in Dijkstra via an era split.
.changes/20260822_035732_cardano-api_palas_dijkstra_tx_construction.yml	Add Herald changelog fragment documenting the new Dijkstra support and the BalanceIsNegative breaking change.

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