expfmt: Add NewTextParser function

[aknuds1]

Add expfmt.NewTextParser function that allows providing name validation scheme. I think this was an omission in #808, since it added the in practice mandatory expfmt.TextParser.scheme field ("Defaults to the invalid UnsetValidation"), but provides no way for clients outside of the expfmt package to configure it. Through expfmt.NewTextParser, prometheus/common clients can safely create expfmt.TextParsers with a valid name validation scheme.

I'm also including a test for NewTextParser.

[Copilot]

Pull Request Overview

This PR adds a NewTextParser function to the expfmt package that allows clients to create TextParser instances with a specified name validation scheme. This addresses an omission where the TextParser.scheme field was made mandatory but no public constructor was provided for external clients to configure it properly.

• Adds NewTextParser(model.ValidationScheme) constructor function
• Updates existing code to use the new constructor instead of zero-value initialization
• Includes comprehensive test coverage for the new function

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
expfmt/text_parse.go	Adds the new NewTextParser constructor function
expfmt/text_parse_test.go	Adds test for the new constructor and updates existing tests to use it
expfmt/fuzz.go	Updates fuzzing code to use the new constructor
expfmt/decode.go	Updates decoder to use the new constructor

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[vesari]

LGTM