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;

[halter73]

@copilot This has conflicts with main. Can you fix that?

[Copilot]

@copilot This has conflicts with main. Can you fix that?

Fixed in 5f8e777. The only conflict was TimeSpanMillisecondsConverter.cs — both branches added the file independently. Resolved by adopting main's more robust implementation (overflow clamping, fractional ms handling, .NET Framework edge cases). Build and all task tests pass.