docs(testing): align aggregation-bits vectors with doc-writer conventions

[tcoratger]

Motivation

Follow-up to #899. That PR added the aggregation-bits bounds vectors, but the documentation did not yet follow the consensus test-vector standard (Given/When/Then with zero body comments). This brings them in line and tightens a few related docstrings.

Changes

Test vectors (tests/consensus/lstar/state_transition/test_aggregation_bits.py)

• Remove all inline body comments; the Given/When/Then docstring is the single source of truth.
• Strip the "a client that…" divergence narration from every Then section so each states only the asserted outcome (rejection reason, or justified/finalized/tally for the valid case).
• Move the bitfield layout and the zero-length-vs-all-false SSZ-encoding rationale into Given as atomic facts.
• Add the bitfield-limit vs validator-registry distinction to the out-of-range vector's Given (index 4 is within the 4096 bitfield limit but past the 4-entry registry).

Framework (packages/testing/.../attestation_specs.py, state_transition.py)

• Collapse the aggregation_bits field and resolver docstrings to terse, one-sentence-per-line phrasing (drop the usage clause).
• Make is_final_block keyword-only on the block builder.

Spec (src/lean_spec/spec/forks/lstar/state_transition.py)

• Note that signature verification normally rejects an out-of-range bit first, so the bounds check guards the unsigned state-transition path.

No behavior change: the four vectors fill and pass unchanged, and the determinism check passes.

Testing

• uv run fill --fork=lstar -k test_aggregation_bits --clean: 4 passed, determinism check passes
• just check: clean