Add gRPC architecture overview

[PureWeen]

Add gRPC architecture overview

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Add an in-area gRPC architecture reference.

Description

Context

#69011 is the wider effort to add expert pull request review guidance. This PR extracts the durable gRPC technical knowledge into an in-area architecture overview that gRPC area owners can validate independently.

The document follows the in-area ARCHITECTURE.md pattern established by @javiercn in #69147. It is a reference for contributors and coding agents that need to understand subsystem ownership and runtime boundaries before making changes.

Summary

• add src/Grpc/ARCHITECTURE.md
• document the ownership boundary between ASP.NET Core integration and the external grpc-dotnet implementation
• describe JSON transcoding endpoint discovery, request binding, invocation, serialization, and error handling
• distinguish native gRPC, HTTP transport, reflection, health checks, code generation, templates, and validation infrastructure
• capture lifetime, concurrency, cancellation, security, and verification boundaries
• map focused documentation and the appropriate subsystem owner for common changes

Documentation only; no runtime or public API changes.

Validation

• git diff --check origin/main...HEAD passed
• all local Markdown links and anchors resolve
• the effective diff contains exactly one new file: src/Grpc/ARCHITECTURE.md
• Markdownlint was not run locally
• runtime tests were not run for this documentation-only change

Related to #69011.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Address the documented accuracy issues in the transcoding flow, option configuration, and startup failure descriptions.

Review tier: Lite
Findings: None

What changed in this PR

Adds an in-area gRPC architecture overview documenting subsystem ownership, JSON transcoding, transport boundaries, and verification infrastructure.

Changes:

• Documents transcoding discovery, binding, invocation, serialization, and error handling.
• Maps ownership across gRPC, ASP.NET Core, protobuf, templates, and tests.
• Describes lifetime, concurrency, cancellation, security, and validation boundaries.

File	Description
src/​Grpc/​ARCHITECTURE.md	Adds gRPC architecture and ownership documentation.

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[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
To make sure no conflicting changes have occurred, please rerun validation before merging. You can do this by leaving an /azp run comment here (requires commit rights), or by simply closing and reopening.

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation accurately reflects the relevant implementation and its local links resolve.

Review effort: Balanced
Findings: None