fix(assert): hint that function properties compare by reference

[LeSingh1]

Fixes #6878.

When assertEquals throws on inputs that only differ in a function property, the diff prints both sides as the same [Function: name] string. The failure then looks like "both sides are identical, why did this throw?".

Repro from the issue:

import { assertEquals } from "jsr:@std/assert";
assertEquals(
  { x: 1, y: () => console.log(2) },
  { x: 1, y: () => console.log(2) },
);

Before:

error: AssertionError: Values are not equal.

    [Diff] Actual / Expected

    {
      x: 1,
      y: [Function: y],
    }

After:

error: AssertionError: Values are not equal.
  Note: function properties are compared by reference, so two distinct
  functions print the same as `[Function: name]` but are not equal.

    [Diff] Actual / Expected

    {
      x: 1,
      y: [Function: y],
    }

Implementation: a small containsFunction walker checks own properties, array entries, Map values, and Set elements (cycle-safe via a WeakSet) and the hint is appended only when at least one operand contains a function. No change to equality semantics or to the diff itself.

Tests cover: the issue's object case, arrays of functions, and a negative case confirming the hint does not appear for function-free inputs. Full assert/ suite (140 tests) passes.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (e0aa7b0).

Files with missing lines	Patch %	Lines
assert/equals.ts	94.44%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7165      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52142    52178      +36     
  Branches     9401     9417      +16     
==========================================
+ Hits        49315    49349      +34     
  Misses       2249     2249              
- Partials      578      580       +2     

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