Set up Copilot instructions (#2325)

* Initial plan

* Add .github/copilot-instructions.md with repository-specific guidance

Co-authored-by: alexeyzimarev <2821205+alexeyzimarev@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexeyzimarev <2821205+alexeyzimarev@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

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+# Copilot Instructions for RestSharp
+
+This document provides instructions for GitHub Copilot when working with the RestSharp repository.
+
+## Project Overview
+
+RestSharp is a lightweight HTTP API client library for .NET. It wraps `HttpClient` and provides:
+- Default parameters of any kind, not just headers
+- Multiple ways to add request parameters (query, URL segment, header, cookie, body)
+- Built-in serialization/deserialization for JSON, XML, and CSV
+- Rich support for authentication
+
+## Repository Structure
+
+- `src/RestSharp/` - Main library
+- `src/RestSharp.Serializers.*/` - Serializer extensions (NewtonsoftJson, Xml, CsvHelper)
+- `gen/SourceGenerator/` - Incremental source generators
+- `test/RestSharp.Tests/` - Unit tests
+- `test/RestSharp.Tests.Integrated/` - Integration tests (WireMock)
+- `test/RestSharp.Tests.Serializers.*/` - Serializer-specific tests
+- `benchmarks/RestSharp.Benchmarks/` - Performance tests
+
+## Build and Test Commands
+
+```bash
+# Build solution
+dotnet build RestSharp.sln -c Debug
+
+# Run all tests
+dotnet test RestSharp.sln -c Debug
+
+# Run tests for specific TFM
+dotnet test test/RestSharp.Tests/RestSharp.Tests.csproj -f net8.0
+
+# Run single test
+dotnet test test/RestSharp.Tests/RestSharp.Tests.csproj --filter "FullyQualifiedName=Namespace.Class.Method" -f net8.0
+
+# Pack for release
+dotnet pack src/RestSharp/RestSharp.csproj -c Release -o nuget -p:IncludeSymbols=true -p:SymbolPackageFormat=snupkg
+```
+
+## Multi-Targeting
+
+The library targets multiple frameworks:
+- `netstandard2.0`, `net471`, `net48` - Legacy support
+- `net8.0`, `net9.0`, `net10.0` - Modern .NET
+
+Tests target: `net48` (Windows only), `net8.0`, `net9.0`, `net10.0`
+
+When adding features:
+- Use conditional compilation for TFM-specific APIs: `#if NET` or `#if NET8_0_OR_GREATER`
+- Ensure compilation succeeds for all TFMs
+- Add polyfills or conditional code for missing APIs on older TFMs
+
+## Code Style and Conventions
+
+- Language version: C# preview (latest features allowed)
+- Nullable reference types: Enabled in `/src`, disabled in `/test`
+- All `/src` files must include the Apache 2.0 license header
+- Test files do not require the license header
+- Follow `.editorconfig` for formatting rules
+- Use partial classes for large types (link with `<DependentUpon>`)
+- Assemblies are strong-named via `RestSharp.snk`
+
+### License Header for Source Files
+
+```csharp
+//  Copyright (c) .NET Foundation and Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+```
+
+## Testing Guidelines
+
+- Frameworks: xUnit + FluentAssertions + AutoFixture
+- Use WireMockServer for HTTP scenarios (avoid live endpoints)
+- Use descriptive assertions: `result.Should().Be(expected)`
+- Guard TFM-specific tests with `#if NET8_0_OR_GREATER`
+- Avoid flaky tests: don't depend on timing, locale, or network conditions
+
+Test projects have global usings configured for `Xunit`, `FluentAssertions`, and `AutoFixture`.
+
+## Source Generators
+
+Custom generators in `gen/SourceGenerator/`:
+- `[GenerateImmutable]` - Creates read-only wrappers
+- `[GenerateClone]` - Creates static factory clone methods
+- `[Exclude]` - Excludes properties from immutable generation
+
+Generator target: `netstandard2.0` (required for all compiler versions)
+
+## Dependencies
+
+Package versions are centrally managed in `Directory.Packages.props`. Do not pin versions in individual projects unless justified by TFM constraints.
+
+## PR Checklist
+
+Before submitting:
+- Builds cleanly across all targeted TFMs
+- Tests added/updated and passing
+- No new analyzer warnings in `/src`
+- License header in new `/src` files
+- Public API changes documented and tested