[Feat]: Allow client generated context id.

[herczyn]

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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