fix(delegation): a present non-integer depth makes a legacy record invalid

[aeoess]

What problem this solves

agent-passport-system/agent-passport-system#223. verify_delegation skipped the depth comparison for a signed maxDepth or currentDepth that is not a number, so the record verified at any depth. The TypeScript verifier coerced "1" to 1 and rejected the same record.

Why this change was made

A verifier should not report a depth result it did not establish, and the two SDKs should agree on validity for the same signed bytes.

User impact

• A present maxDepth or currentDepth must be a non-negative integer no greater than 2**53 - 1, matching the TypeScript safe-integer limit. Anything else makes the record invalid, with a Malformed maxDepth or Malformed currentDepth error. depthExceeded stays False because no comparison was made.
• Integer-valued floats such as 2.0 remain accepted, since they sign to the same bytes as 2.
• An absent or None member is not compared, matching the TypeScript verifier.
• _is_depth moves to module level, so verify_delegation, sub_delegate and create_delegation share one depth rule.
• create_delegation now raises ValueError for a max_depth its verifier would reject, so the SDK cannot return a signed record its own verifier rejects. None is unchanged: it signs as no maxDepth member, which verifies with no ceiling.
• The TypeScript side is fix(delegation): a present non-integer depth makes a legacy record invalid agent-passport-system#225 with the same rule.

Evidence

• tests/test_legacy_maxdepth_absent.py: the test that pinned the old skip is replaced. Malformed maxDepth at depth 0 and 2, malformed currentDepth with and without a ceiling, integer-valued floats, None and absent members. With the verifier change reverted, the new cases fail.
• Writer tests: 1.5, -1, 2**53, NaN, infinity, "1" and True raise, and 0, 1, 2.0, 2**53 - 1 and None sign and verify. With the writer guard reverted, the refusal test fails.
• maxDepth 2**53 is malformed and 2**53 - 1 is accepted, in both SDKs.
• pytest: 2756 passed, 6 skipped.

Boundaries

• Legacy delegation verification only. No change to the v2 path or to the absent maxDepth reading from fix(delegation): no depth ceiling for an absent maxDepth, refuse minting without a ceiling #25.