Slim the mqttea error tests and table-ify dpf's node-label checks

[chet]

mqttea's tests/errors.rs grew a large block of overlapping and low-value tests; dpf has a per-case node-label cluster and two duplicate config tests. One concern per table, and the tests that only exercise the stdlib or a timing threshold can go. Test-only; no behavior change.

Scope

mqttea

• Collapse tests/errors.rs (~38 tests): the overlapping from-conversion, categorization, and Display / source duplicates fold into one check_cases table per concern (roughly a dozen functions removed).
• Delete the assert_send / assert_sync compile-check tests and the Result / Option ergonomics tests (they exercise the stdlib, not our error type).
• Delete the two timing/performance assertions (errors.rs and stats.rs); move to a bench if the measurement is wanted.

dpf

• Collapse the six verify_node_labels_* tests into one check_cases_async table (sdk.rs).
• Delete test_init_config_defaults (duplicates the types.rs defaults table) and test_init_config_custom (re-asserts fields it just set).
• Merge dpu_phase_display_matches_as_ref into its as_ref sibling (types.rs).

Acceptance

• Every error variant / node-label case still covered; the files shrink substantially with no coverage loss.
• cargo test green for both crates; cargo clippy --all-features clean.

Size: M

Part of #2692.