fix(test): props batch-size alias test assumes a 512 default prefill chunk #2211

Problem / Background

Since PR #2205 (one prefill plan and one trim hook for the CLI and the server, part of epic #2166), server::routes::props::props_tests::geometry_block_reports_the_resolved_batch_size_alias (src/server/routes/props_tests.rs:231) fails deterministically with left: 512, right: 1024. It is backend-independent.

The test calls resolve_prefill_chunk_size(512, Some(1024), Some(256)) and expects --batch-size 1024 to win, assuming 512 is the default --prefill-chunk-size. #2205 made resolve_prefill_chunk_size (src/server/cli_input.rs:2276) compare against mlxcel_core::prefill_plan::prefill_chunk_len(), whose default is DEFAULT_PREFILL_CHUNK = 2048 (overridable by MLXCEL_PREFILL_CHUNK). So 512 now counts as an explicit chunk and the alias does not apply.

Production behavior is correct per ADR 0007 (docs/adr/0007-unified-batch-native-engine.md, "Prefill chunk" row: one default of 2048 for the CLI and the server). The test is stale; src/server/cli_input_tests.rs already uses prefill_chunk_len() for the default.

Proposed Solution

• Make the props test pass prefill_chunk_len() as the default argument instead of the literal 512.
• Correct the comment in src/bin/bench_engine.rs:87 that says the server default is 512.
• Correct the comment in src/models/phi3_tests.rs:211. Its test uses a 512 chunk as a pinned fixture, which stays valid.

Acceptance Criteria

• The test derives the default from prefill_chunk_len().
• Stale 512 default comments are corrected.
• cargo test --profile test-fast --features rocm -p mlxcel --lib server::routes::props and server::cli_input pass.