Treat output from TextMarshaler as string

[shuheiktgw]

Closes #689.

The original issue was that since null.StringFrom implements the TextMarshaler interface, goccy/go-yaml tries to interpret its output as a YAML document using e.encodeDocument. However, because the output contains a special character (in this case, #), the parser fails to handle it correctly and returns nil.

IIUC, the output from a TextMarshaler implementation should not be interpreted as a YAML document. It should be treated as a plain string, since TextMarshaler is intended to return a textual representation of a value. This behavior is consistent with gopkg.in/yaml.v3.

One side effect I noticed after making this change is related to how *time.Time is encoded. Previously, since *time.Time also implements TextMarshaler, it was being encoded by the code block below. After treating the output from MarshalText() as a string, the library started wrapping *time.Time values in double quotes. To prevent this, I updated the encodeByMarshaler function to handle *time.Time explicitly.

if marshaler, ok := iface.(encoding.TextMarshaler); ok {
		doc, err := marshaler.MarshalText()
		if err != nil {
			return nil, err
		}
		node, err := e.encodeDocument(doc)
...
}

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.92%. Comparing base (d1e3460) to head (a75057e).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
+ Coverage   77.89%   77.92%   +0.03%     
==========================================
  Files          22       22              
  Lines        7965     7965              
==========================================
+ Hits         6204     6207       +3     
+ Misses       1348     1346       -2     
+ Partials      413      412       -1     

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[goccy]

This works correctly 👍

Since isInvalidValue becomes true at the beginning of encodeValue when *time.Time is nil, this process is always executed only when *time.Time is not nil.

[goccy]

Thank you for your great PR ! LGTM 👍