feat(EVM-1116): [AI] document observable bytecode len

[0xVolosnikov]

What ❔

• Replaced the TODO in account cache entry with a clear explanation of observable_bytecode_len semantics.
• Documented why this value can differ from historical account metadata lengths.

Why ❔

• Clarifies an important invariant in account metadata handling.
• Reduces ambiguity in a security-sensitive code path.

Is this a breaking change?

• Yes
• No

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted.

[github-actions]

Benchmark report

Benchmark	Symbol	Base Eff	Head Eff (%)	Base Raw	Head Raw (%)	Base Blake	Head Blake (%)	Base Bigint	Head Bigint (%)
block_19299001	process_block	315,681,952	315,681,952 (+0.00%)	273,016,552	273,016,552 (+0.00%)	410,630	410,630 (+0.00%)	9,023,830	9,023,830 (+0.00%)
block_22244135	process_block	197,585,908	197,585,908 (+0.00%)	170,670,116	170,670,116 (+0.00%)	172,040	172,040 (+0.00%)	6,040,788	6,040,788 (+0.00%)
precompiles	bn254_ecadd	53,268	53,268 (+0.00%)	47,816	47,816 (+0.00%)	0	0 (+0.00%)	1,363	1,363 (+0.00%)
precompiles	bn254_ecmul	728,781	728,781 (+0.00%)	564,593	564,593 (+0.00%)	0	0 (+0.00%)	41,047	41,047 (+0.00%)
precompiles	bn254_pairing	72,336,733	72,336,733 (+0.00%)	57,808,589	57,808,589 (+0.00%)	0	0 (+0.00%)	3,632,036	3,632,036 (+0.00%)
precompiles	ecrecover	479,714	477,458 (-0.47%)	311,130	309,758 (-0.44%)	0	0 (+0.00%)	42,146	41,925 (-0.52%)
precompiles	id	927	927 (+0.00%)	927	927 (+0.00%)	0	0 (+0.00%)	0	0 (+0.00%)
precompiles	keccak	137,579	137,579 (+0.00%)	137,579	137,579 (+0.00%)	0	0 (+0.00%)	0	0 (+0.00%)
precompiles	modexp	31,267,857	31,267,857 (+0.00%)	20,610,037	20,610,037 (+0.00%)	0	0 (+0.00%)	2,664,455	2,664,455 (+0.00%)
precompiles	p256_verify	748,861	748,861 (+0.00%)	470,169	470,169 (+0.00%)	0	0 (+0.00%)	69,673	69,673 (+0.00%)
precompiles	point_evaluation	52,918,413	52,918,413 (+0.00%)	40,570,377	40,570,377 (+0.00%)	0	0 (+0.00%)	3,087,009	3,087,009 (+0.00%)
precompiles	process_block	147,545,393	147,556,574 (+0.01%)	118,077,157	118,086,406 (+0.01%)	5,130	5,160 (+0.58%)	7,346,539	7,346,902 (+0.00%)
precompiles	process_transaction	73,489,153	73,491,462 (+0.00%)	58,792,681	58,794,178 (+0.00%)	160	160 (+0.00%)	3,673,478	3,673,681 (+0.01%)
precompiles	ripemd	8,013	8,013 (+0.00%)	8,013	8,013 (+0.00%)	0	0 (+0.00%)	0	0 (+0.00%)
precompiles	run_tx_loop	146,899,996	146,904,853 (+0.00%)	117,510,960	117,514,365 (+0.00%)	180	180 (+0.00%)	7,346,539	7,346,902 (+0.00%)
precompiles	sha256	13,168	13,168 (+0.00%)	13,168	13,168 (+0.00%)	0	0 (+0.00%)	0	0 (+0.00%)
precompiles	system_init	46,785	46,785 (+0.00%)	46,785	46,785 (+0.00%)	0	0 (+0.00%)	0	0 (+0.00%)
precompiles	verify_and_apply_batch	146,508	146,508 (+0.00%)	110,188	110,188 (+0.00%)	2,270	2,270 (+0.00%)	0	0 (+0.00%)