feat: FlexiRaft for even-sized data commit quorums

[lshir]

Proposed Changes

Replaces #638 and #642 - implementing only the data commit quorum optimization for even clusters described in #500

For even-sized clusters, we can commit an entry with N div 2 rather than N div 2 + 1 acks. This change is only applied to clusters that are not changing in size, and we do not touch the query quorum path. We also only apply for clusters with N>=4 so that a single node does not constitute a quorum.

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[Copilot]

Pull request overview

Implements the FlexiRaft optimization for data commit quorums in even-sized clusters by allowing commits with N div 2 (instead of classic majority N div 2 + 1) when cluster membership is stable, while keeping election and query quorums unchanged.

Changes:

• Adjusts leader commit-index advancement to use a reduced data commit quorum for even voter counts when cluster_change_permitted = true.
• Adds/updates unit and integration tests covering the even-quorum commit behavior and related quorum helpers.
• Documents the flexible quorum behavior in internal docs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/ra_SUITE.erl	Fixes a minor comment typo in an existing test.
test/ra_server_SUITE.erl	Adds an even-quorum commit test and updates existing expectations/ordering in leader handling tests.
src/ra_server.erl	Introduces data_commit_quorum_size/2 and agreed_commit/2 to support reduced data commit quorums for even voter counts.
docs/internals/INTERNALS.md	Adds internal documentation describing the flexible quorum optimization and when it is applied.

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