fix(mcp): send search via POST and preflight oversized store payloads

[bharatindia123-a11y]

Summary

• switch the MCP memory_search client from GET /search to POST /search with a JSON body
• preserve limit clamping while avoiding long-query URL overflow / HTTP 414 failures
• add an MCP-side preflight guard for memory_store content above 8000 chars so oversized requests fail clearly before transport
• update MCP tests to verify POST search payloads, collection propagation, and the size guard

Why

The core Rasputin server already supports POST /search and intentionally enforces the 8000-char /commit limit. The correct fix in this repo is therefore in the MCP client layer rather than the server core.

Test Plan

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest -o addopts='' tests/test_mcp.py -q

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