test: cover collecting a subtree of errors and grouping a nested anyOf

[alexander-akait]

Follow up to #217, which merged before this landed.

codecov reported 5 lines of #217 without coverage. Four of them are in the instance path index
that filterErrors uses above 24 errors: it was only exercised by errors that share an instance
path or have none in common, so the branch that walks into the children of a node and the comparator
that puts the collected errors back in the order they were reported were never reached.

An error that arrives after the errors nested inside it collects a whole subtree at once and covers
both - a value failing an anyOf whose object branch reports an error per property:

{ a: { anyOf: [{ type: "object", properties: { p0: …, p29: … } }, { type: "string" }] } }

The second test covers groupChildrenByFirstChild putting the errors of an inner anyOf under it
rather than listing every branch flat, which is the path findAllChildren was changed for in #217.

src/validate.js goes from 97.84% to 100% of lines, statements and functions. What is left there is
branch coverage for the guards of environments without process or globalThis, which cannot be
taken on the Node.js the tests run on.

Tests only, no source changes.

🤖 Generated with Claude Code

https://claude.ai/code/session_013wqLmVHkAGBsWQgXknEQCK

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.78%. Comparing base (17c4e3a) to head (1d45c17).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   95.36%   95.78%   +0.42%     
==========================================
  Files           9        9              
  Lines         949      949              
  Branches      387      387              
==========================================
+ Hits          905      909       +4     
+ Misses         40       36       -4     
  Partials        4        4              

Flag	Coverage Δ
integration	95.78% <ø> (+0.42%)	⬆️

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