Cleanup exception throwing in Utf8JsonWriter

[pentp]

• Remove some unused throw helpers and exception strings.
• Make all throw helpers visible as not-returning to JIT so that the callsites can be properly optimized.
• Refactor some duplicated throw helpers.

This addresses #111332 (comment)

[Copilot]

Pull Request Overview

This PR refactors exception throwing helpers in Utf8JsonWriter and related components to remove unused helpers, improve JIT optimization by marking throw methods as not-returning, and consolidate duplicated code.

• Refactored throw helper methods to use new patterns (e.g. GetValidateStartFailedException)
• Replaced ValidatePropertyAndDepth calls with separate property and depth validations
• Updated Utf8JsonReader and MultiSegment handling to follow the new exception patterns

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.cs	Refactored property getters and exception throwing methods
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Raw.cs	Updated calls to throw helpers for raw value validation
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Helpers.cs	Consolidated validation helpers for writing values
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.String.cs	Updated property and value validations in string writing methods
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Helpers.cs	Replaced ValidatePropertyNameAndDepth with separate validations
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Bytes.cs	Replaced property name length validation calls with updated methods
src/libraries/System.Text.Json/src/System/Text/Json/Writer/JsonWriterHelper.cs	Removed unused composite validation methods
src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.cs	Removed redundant throw helpers and updated exception construction patterns
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.cs	Updated literal validation to use new exception patterns
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.MultiSegment.cs	Adjusted multi-segment literal validation to use scoped Span parameters
src/libraries/System.Text.Json/src/Resources/Strings.resx	Removed obsolete string resources

[eiriktsarpalis]

Thanks, but I think I need to better understand what the motivation for filing this PR is. As it stands I think this should be split into at least two separate PRs (one eliminating dead code, and one proposing changes to the helpers) so that each can be reviewed in it own merits.

[pentp]

There's not much dead code actually, only the 3 strings (and 3 throw helpers). Everything else is about removing NoInlining from throw helpers and making them small enough that the JIT actually sees that they are throwing (never returning) and optimizes all callsites as cold code.
This gives significant code size wins, I can try running the STJ benchmarks also.

[jkotas]

I can try running the STJ benchmarks also.

Yes, we like to see numbers for any perf related changes.

[pentp]

I was waiting on #115918, but I guess I could get some perf numbers regardless of that issue.

[pentp]

I addressed the feedback, but this PR now depends on #118280 getting merged first. Will gather some perf numbers also now.

[pentp]

@MihuBot benchmark System.Text.Json