fix: MCP toolset resolution for runtime and config

[teknium1]

Summary

Salvage of #3203 by @kshitijk4poor with follow-up fixes.

MCP server tools (e.g. exa, web-search-prime) were discovered and registered in the tool registry, but never made it into the model's tool list. The gateway had its own inline toolset resolution that only read platform_toolsets from config — which never contains MCP server names. Same issue affected the CLI entry point.

Changes

From #3203 (cherry-picked, author preserved):

• Replace duplicated gateway toolset resolution in _run_agent() and _run_background_task() with calls to the shared _get_platform_tools()
• Extend _get_platform_tools() to include globally enabled MCP servers at runtime (include_default_mcp_servers=True), while config-editing flows pass False to avoid persisting implicit MCP defaults
• Add homeassistant to PLATFORMS dict (was missing, caused KeyError)
• Consolidate duplicate config YAML reads in gateway
• Add regression tests for MCP inclusion, HomeAssistant, and config persistence

Follow-up fixes:

• Remove redundant platform_key reassignment in _run_background_task()
• Fix CLI entry point (cli.py) to also use _get_platform_tools() — MCP tools were invisible in CLI mode too

Tests

Full suite: 6209 passed, 1 pre-existing failure (unrelated test_429_exhausts_all_retries_before_raising).

Closes #3203

[github-actions]

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