fix(core): write null Task.ttl instead of omitting it

[devcrocod]

Tasks with unlimited retention now go on the wire with "ttl": null: the schema requires the key, and McpJson used to drop it.

closes #1078

How Has This Been Tested?

• Flipped the TasksTest and NotificationTest cases that asserted the omission, and added cases for GetTaskResult with a null ttl, TaskStatusNotification.toJSON() and decoding a task without ttl. The serialization cases fail without the fix.
• kotlin-sdk-core JVM tests, the types tests on JS (Node), Wasm (Node) and macosArm64, plus ktlint, detekt and checkKotlinAbi.
• Checked the encoded CreateTaskResult, GetTaskResult, CancelTaskResult, ListTasksResult and notifications/tasks/status against the TypeScript SDK 1.30.1 schemas: they pass with "ttl": null and fail without the key.

Breaking Changes

None. The public API is unchanged; ttl is now always present on the wire.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation consistently covers affected wire types with focused regression tests and preserves decoding compatibility.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes task serialization to emit required nullable ttl fields, aligning with the MCP 2025-11-25 schema.

Changes:

• Adds shared serializers that restore "ttl": null.
• Applies them to all task-state wire types.
• Expands serialization and backward-compatible decoding tests.

File	Description
tasks.kt	Applies serializers to task result types.
notification.kt	Applies serialization to status notification parameters.
serializers.kt	Adds the task-field transforming serializer.
TasksTest.kt	Tests task results, lists, and legacy decoding.
NotificationTest.kt	Tests notification and JSON-RPC conversion.

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[jetbrains-air]

Nice, clean fix — this looks good to merge, thanks!

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[e5l]

lgtm

[Copilot]

🟢 Approval recommended

The implementation consistently addresses all affected task wire formats with appropriate regression coverage.

0 open findings

🧠 Review effort: Balanced

[jetbrains-air]

Looks good — happy to approve this one, thanks!

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[jetbrains-air]

Documentation maintenance — one stale statement, in a file outside this diff (so it could not be anchored inline).

AGENTS.md:69 (also reachable as CLAUDE.md, a symlink to it)

Stale statement

• McpJson has explicitNulls = false: null properties must be absent from JSON, not "field": null

It is written as an absolute rule: no null property may ever appear on the wire.

The change that falsified it

This PR put Task, GetTaskResult and TaskStatusNotificationParams behind the new TaskFieldsSerializer (@Serializable(with = …) + @KeepGeneratedSerializer), which re-adds "ttl" as JsonNull after McpJson has dropped it. TasksTest and NotificationTest now assert "ttl": null in the encoded output, so the repo's own tests contradict the rule as stated. An agent reading line 69 as written would treat "ttl": null as a violation and revert the fix for #1078.

Suggested correction

Keep the rule as the default and record the schema-mandated exception:

- **`McpJson` has `explicitNulls = false`**: null properties are absent from JSON, not
  `"field": null` — except where the MCP schema requires the key, which a
  `JsonTransformingSerializer` has to re-add (see `TaskFieldsSerializer` for `Task.ttl`)

Nothing else went stale: the ttl KDoc on TaskFields, Task, GetTaskResult and TaskStatusNotificationParams ("null for unlimited") still matches the code, and neither the README, /docs, the Module.md files nor the sample READMEs document tasks or ttl.

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