Widen model config to use JSON primitive

[pwang347]

No description provided.

[pwang347]

@copilot help me add changelog entries

[Copilot]

@copilot help me add changelog entries

Added ### Changed entries under ## [Unreleased] in all six CHANGELOGs (spec + all 5 clients) in commit $(git -C /home/runner/work/agent-host-protocol/agent-host-protocol rev-parse --short HEAD). Each entry notes that ModelSelection.config values were widened from string to the language-appropriate JSON primitive type.

@copilot help me add changelog entries

Added ### Changed entries under ## [Unreleased] in all six CHANGELOGs (spec + all 5 clients) in commit 9921c52. Each entry notes that ModelSelection.config values were widened from string to the language-appropriate JSON primitive type.