[Feat]: Allow client generated context id.

### Is your feature request related to a problem? Please describe.

In a multi-agent deployment it's easier to keep the mapping on the server side, and some agent frameworks just allow providing session id so there is no need for mapping.

### Describe the solution you'd like

Remove wording around context id being server generated and add add "accepts_client_generated_context_id" to agent capabilities.

### Describe alternatives you've considered

1) leave as is
this makes some things complicated. for instance:

` A `
` / \ `
` \/ \/`
` B C`
` \ /`
` \/\/`
` D `

if we want to keep the same context in calls to D it is overly complex and in some cases impossible (B receives the initial context id, how is it supposed to pass it to C for reuse)?

2) use a new extension to "hint" the context id
this will work, but i think context id is so embedded in the spec, that i think this should be more explicit

### Additional context

_No response_

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