partitions: refactor inline tests #4684

Work can start only after both PR #4683 and PR #4658 have merged.

Separate tests from implementation as the first step toward refactoring core/partitions/. At c92d2bd907cf, iggy_partition.rs contains 35,344 lines, roughly 21,800 of them in inline test modules. Large suites also live in segment_recovery.rs and state_transfer.rs, making production code harder to navigate and review.

Design

Start with those three files and follow the message_bus layout:

• Move tests using the existing public API to core/partitions/tests/, grouped by behavior. Extend the layout established by consumer_offset_history.rs.
• Extract private unit suites into separate child module files under src/, retaining their #[cfg(test)] declarations and fixture access.
• Share reused external fixtures through tests/common/mod.rs; keep specialized helpers with their suites.

Keep production code and visibility unchanged. Preserve assertions, feature guards, fault injection, runtime setup and fixture lifetimes. Leave unrelated unit suites alone.

Alternatives

A wholesale move to tests/ would require exposing internals or rewriting tests: some directly mutate partition.log or call helpers available only under cfg(test). Separate private unit modules preserve that access.

Risks

Relocation can lose test discovery or alter setup and cleanup. All-features builds can also hide dependence on simulator or fault-injection APIs, so validate default features independently.

Tests

• Large inline suites are extracted from the three target files.
• Every existing test has a destination; compare discovered test identities before and after, allowing for module path changes.
• No coverage is weakened or tests newly ignored; default-feature and all-features suites pass.
• Public APIs, runtime behavior, storage formats and durability ordering remain unchanged.

Follow-ups

Split IggyPartition responsibilities in subsequent changes, reusing existing modules such as persistence.rs, poll_plan.rs, io.rs and state_transfer.rs.

Commands

Compare test discovery before and after for each configuration, then run the repository checks:

rtk cargo test -p partitions -- --list
rtk cargo test -p partitions --all-features -- --list
rtk cargo fmt --all
rtk cargo sort --no-format --workspace
./scripts/ci/taplo.sh --check
rtk cargo build
rtk cargo clippy --all-features --all-targets -- -D warnings
rtk cargo test -p partitions
rtk cargo test -p partitions --all-features