STJ avoid full document string allocation

[eMelgooG]

Refs: #4652 (comment)

Pull Request Template

Description

Both read paths in CosmosSystemTextJsonSerializer currently materialize the entire response document as a UTF-16 System.String before parsing. This PR feeds the UTF-8 bytes directly into Utf8JsonReader instead, eliminating the allocation and the redundant UTF-8 ↔ UTF-16 transcoding on every read.

Binary path: replace cosmosObject.ToString() (which produces a UTF-16 string) with cosmosObject.WriteTo(textJsonWriter) + JsonSerializer.Deserialize<T>(ReadOnlySpan<byte>, ...).

Text path (DeserializeStream<T>): replace StreamReader.ReadToEnd() + Deserialize<T>(string, ...) with JsonSerializer.Deserialize<T>(Stream, ...).

Both changes are semantically equivalent for Cosmos response bodies (UTF-8 JSON), no public API impact. Reduces CPU, allocations, and GC/LOH pressure on every read.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Closing issues

n/a

Changelog

• [X ] I have added a changelog entry under ### Unreleased in changelog.md
  for the user-facing impact of this change.

[eMelgooG]

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