feat: add agent/model/variant pre-select through url parameters

[damejeras]

What does this PR do?

This adds a support for agent, model and variant pre-selection through URL parameters in /session endpoint.
This allows better integration with external tools. For example neovim users could bind a keybinding to open new session on a project with Plan agent and specific model pre-selected.

Closes: #6822

For example
http://127.0.0.1:62136/QzpcVXNlcnNcYWRtaW5cQ29kZVxvcGVuY29kZS1wbHVnaW4/session?agent=plan will pre-select Plan agent in web UI.

additional URL parameters:
?model= (example ?model=opencode/gpt-5-nano)
?variant= (example ?variant=high)

How did you verify your code works?

I have tested it manually by entering URL in a browser and seeing parameters in the chat box being selected.

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Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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