fix: pin fast-agent-mcp==0.7.15 to match the 0.7.15-era API

pallas 0.5.0 targets fast-agent 0.7.15 but pinned it unbounded
(>=0.7.15). Deploys install with --upgrade --force-reinstall, so pip
pulled 0.9.9, whose major refactor removed fast_agent.core.prompt,
fast_agent.mcp.server.AgentMCPServer (the MultimodalAgentMCPServer base
class), and the mcp_aggregator/mcp_agent_client_session re-exports —
crashing every consumer (kottos, mentor, iolaus) at import time.

Pin exactly until pallas is ported to the 0.9.x Harness* API.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -1,10 +1,10 @@
 [project]
 name = "pallas-mcp"
-version = "0.5.0"
+version = "0.5.1"
 description = "FastAgent MCP Bridge — generic runtime for serving FastAgent agents over StreamableHTTP"
 requires-python = ">=3.13.5"
 dependencies = [
-    "fast-agent-mcp>=0.7.15",
+    "fast-agent-mcp==0.7.15",
     "httpx",
     "prometheus-client",
     "pyyaml",