Add per-call SerializationContext overloads

[AArnott]

Why

Advanced callers need per-operation converter state and settings without creating a separate serializer for every call. Adding a context parameter to every overload would unnecessarily multiply the public API surface.

What changed

• Add context-taking overloads for the core sync and async typed and untyped serialize/deserialize entry points (MessagePackWriter/MessagePackReader and PipeWriter/PipeReader). Existing overloads preserve their signatures and delegate using StartingContext; the context-taking overloads use the supplied context and its cancellation token.
• Share implementation through private cores. Sync cores initialize the context by reference to avoid an additional large-struct copy. Give the new overloads lower overload-resolution priority so explicit default continues to bind to the existing cancellation-token overloads.
• Reject contexts already active in another serialization operation, directing converters to SerializationContext.GetConverter for nested values. Add a protected CreateSerializationContext overload accepting a per-call context and move internal callers away from the overload that implicitly uses StartingContext.
• Document per-call state and add a sample and tests.

Validation

• Build/pack and formatting verification pass; DocFX builds; AOT console sample ends with Success.
• The 13 new tests pass on net8.0, net9.0, net10.0, and net472.
• The full test run has 19 net472 failures involving Newtonsoft.Json.Linq.JToken.WriteTo MissingMethodException; the same 19 failures reproduce with the changes stashed.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation and tests are coherent; remaining feedback concerns minor documentation and message accuracy.

Review effort: Balanced
Findings: 3

Open (3)

• Use DocFX xref syntax for the reference · New
• Document rejection of previously initialized contexts · New
• Clarify previously initialized contexts cannot be starting contexts · New

What changed in this PR

Adds per-call SerializationContext support across core serialization APIs.

Changes:

• Adds sync/async typed and untyped context overloads.
• Rejects previously initialized contexts and consolidates shared implementation.
• Adds documentation, samples, and comprehensive tests.

File	Description
MessagePackSerializer.cs	Adds context overloads and shared cores.
SerializationContext.cs	Adds initialized-context validation.
PerCallSerializationContextTests.cs	Tests context state, cancellation, depth, and overload resolution.
CustomConverters.cs	Demonstrates per-call state.
custom-converters.md	Documents the new API.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is consistent across entry points and adequately tested and documented.

Review effort: Balanced
Findings: None

Resolved since last review (3)

• Clarify previously initialized contexts cannot be starting contexts
• Document rejection of previously initialized contexts
• Use DocFX xref syntax for the reference

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation, validation, documentation, and tests consistently cover the new per-call context behavior.

Review effort: Balanced
Findings: None

[codecov]

Codecov Report

❌ Patch coverage is 94.89796% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.27%. Comparing base (15f3ca3) to head (1fffefc).

Files with missing lines	Patch %	Lines
src/Nerdbank.MessagePack/MessagePackSerializer.cs	92.53%	8 Missing and 2 partials ⚠️
...sagePack.TUnit/PerCallSerializationContextTests.cs	96.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1085      +/-   ##
==========================================
+ Coverage   82.01%   82.27%   +0.25%     
==========================================
  Files         256      257       +1     
  Lines       20866    21057     +191     
  Branches     2794     2795       +1     
==========================================
+ Hits        17113    17324     +211     
+ Misses       2864     2845      -19     
+ Partials      889      888       -1     

Flag	Coverage Δ
Linux	82.01% <94.89%> (+0.20%)	⬆️
Windows	82.10% <94.89%> (+0.10%)	⬆️
macOS	82.02% <94.89%> (?)

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