[v1.x] fix: disallow null (infinite) requested TTL

[LucaButBoring]

The specification only allows the server to return a null TTL, it doesn't allow the client to request that. This is intentional, as it forces clients to make a choice betwen a specific TTL or not caring what the TTL is, rather than defaulting to keeping tasks forever all the time.

This change removes null as a valid requested TTL. Allowing the requested value to be null was an implementation oversight that diverged from the spec.

Motivation and Context

Fixes a request shape mismatch against the specification.

How Has This Been Tested?

Updated unit tests.

Breaking Changes

Yes, if anyone was requesting a null TTL.

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

Additional context

Partial backport of #1315.

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Latest commit: f51932f

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commit: f51932f