fix(assert): hint at reference-equality when assertEquals diff is empty (#6878)

[LeSingh1]

Fixes #6878.

When assertEquals decides two values are unequal but they stringify identically (typically because a property is a function compared by reference), the rendered diff is empty:

error: AssertionError: Values are not equal.


    [Diff] Actual / Expected


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

That leaves users hunting for invisible differences. @lionel-rowe pointed at the underlying cause in the thread (functions, Promises, Requests, Blobs etc. fall back to reference equality) and suggested the diff should "explicitly state when there are invisible differences."

Fix

After building the diff, check whether it contains any added / removed entries. If not, append:

    Note: values stringify identically but are not structurally equal.
    Functions, Promises, Requests, Blobs, and other built-ins are compared by
    reference, so two distinct instances are never equal even when their
    representations match.

The hint is skipped on string-vs-string comparisons (which always produce a visible diff when they differ) and on real textual diffs (so existing failures aren't noisier).

Tests

Two cases in assert/equals_test.ts:

• hints at reference-equality when objects with function props stringify identically (#6878) — reproduces the exact issue body's call and asserts the new hint text is present.
• does not append reference-equality hint when there is a real textual diff — assertEquals({ x: 1 }, { x: 2 }) must NOT include the hint.

$ deno test --allow-read --allow-write assert/
ok | 141 passed (25 steps) | 0 failed (1s)

$ deno lint assert/equals.ts assert/equals_test.ts
Checked 2 files

$ deno check assert/equals.ts assert/equals_test.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (f0c9f14) to head (c379203).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7158   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files         636      636           
  Lines       52138    52149   +11     
  Branches     9399     9403    +4     
=======================================
+ Hits        49311    49322   +11     
  Misses       2249     2249           
  Partials      578      578           

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