Use TimeToLive (TimeSpan?) instead of TtlMs (long?) in public APIs

[Copilot]

Ttl abbreviation in public APIs is hard to read. This PR spells it out as TimeToLive and also changes the type from long? (milliseconds) to TimeSpan? for a more idiomatic .NET API. A new TimeSpanMillisecondsConverter handles JSON serialization so the wire format is unchanged.

Changes

New type

• TimeSpanMillisecondsConverter — a JsonConverter<TimeSpan> that reads/writes millisecond integers on the wire

Renamed and retyped identifiers

Type	Before	After
McpTaskInfo	long? TtlMs	TimeSpan? TimeToLive
CreateTaskResult	long? TtlMs	TimeSpan? TimeToLive
GetTaskResult	long? TtlMs	TimeSpan? TimeToLive
TaskStatusNotificationParams	long? TtlMs	TimeSpan? TimeToLive
InMemoryMcpTaskStore	long? DefaultTtlMs	TimeSpan? DefaultTimeToLive

The JSON wire format property name ("ttlMs") is unchanged — only C# identifiers and types are affected.

// Before
var store = new InMemoryMcpTaskStore { DefaultTtlMs = 30_000 };
long? ttl = result.TtlMs;

// After
var store = new InMemoryMcpTaskStore { DefaultTimeToLive = TimeSpan.FromSeconds(30) };
TimeSpan? ttl = result.TimeToLive;

[PranavSenthilnathan]

This should just be a TimeSpan? named TimeToLive and it should use the custom converter TimeSpanMillisecondsConverter. Same for the others as well.

[Copilot]

Done. Changed all five occurrences (CreateTaskResult, GetTaskResult, TaskStatusNotificationParams, McpTaskInfo, and InMemoryMcpTaskStore.DefaultTimeToLive) from long? to TimeSpan?, introduced TimeSpanMillisecondsConverter for JSON serialization (the wire format "ttlMs" number is preserved), and updated all usages and tests.