fix(ini): quote ambiguous string values in stringify to preserve round-trip type

[LeSingh1]

Fixes #4809.

When you call stringify on an object whose values are strings that happen
to look like another type, the output loses type information. For example:

stringify({ key: "true" })  // produces: key=true
parse("key=true")           // returns:  { key: true }  (boolean, not string)

The same happens with "false", "null", and any numeric string like "123".
The string is written bare, and parseValue then converts it back to the
wrong type. A round-trip through parse(stringify(obj)) silently changes
"true" to true, "null" to null, "123" to 123, and so on.

The root cause is in defaultReplacer, which just does template literal
interpolation without checking whether the string value would be
misread by parseValue. parseValue already supports double-quoted strings
via QUOTED_VALUE_REGEXP and returns them as-is. So the fix is to have
defaultReplacer wrap the ambiguous strings in double quotes.

Plain strings that cannot be mistaken for another type (like "hello" or
"123foo") are left unquoted as before.

I added assertions for the individual quoted outputs and a round-trip test
covering all the affected cases. All 18 tests pass, deno fmt and deno lint
are clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (15ed008).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7182      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      637       +1     
  Lines       52142    52160      +18     
  Branches     9401     9408       +7     
==========================================
+ Hits        49315    49329      +14     
- Misses       2249     2254       +5     
+ Partials      578      577       -1     

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