Agents: support multiple pull requests per session (#333189)

* sessions: support multiple pull requests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: cover multi-PR review loading

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: address multi-PR review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: register banner fixture services

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: register toolbar fixture providers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: align input banner split buttons

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Dmitriy Vasyura <dmitriv@microsoft.com>