FINERACT-2485: Align legacy idempotency key storage width

[mcc0nnell]

Core Fineract currently accepts the configured idempotency header without applying a length constraint before command processing. The legacy m_portfolio_command_source.idempotency_key column and its JPA mapping are limited to 50 characters, while the newer JDBC command store uses VARCHAR(255).

This change keeps the two command-storage paths consistent by:

• widening m_portfolio_command_source.idempotency_key from 50 to 255 characters via a new Liquibase migration
• updating the CommandSource JPA mapping to match
• leaving existing idempotency behavior and keys unchanged

This is intentionally separate from #6231, which is hardening idempotency semantics for the newer command-processing path. This patch only removes a storage-width inconsistency in the legacy path.

[IOhacker]

@mcc0nnell please resolve the conflict

[mcc0nnell]

@IOhacker Conflict is resolved on the current signed head df563d9, and GitHub now reports the PR as mergeable. The resolution preserves upstream migrations 0254/0255 and renumbers this PR's idempotency-key migration to 0256; the JPA mapping remains length = 255. The new Fineract CI, PR Compliance, and Signed Commits Check runs are all blocked at GitHub's fork-workflow action_required approval gate with no jobs started. Could you please approve and run the workflows?

[adamsaghy]

@mcc0nnell Please make sure your commits are GPG signed.