fix(base): reference-count session lifecycle for safe concurrent use

[codemug]

Problem

Under concurrent MCP requests, tools were failing with:

WARNING:aiofmp.base:HTTP client error, attempt 2/4: Connector is closed.
ERROR:aiofmp.company_tools:Error in get_shares_float: 'NoneType' object has no attribute 'get'
ERROR:aiofmp.company_tools:Error in get_company_profile: 'NoneType' object has no attribute 'get'

This was not an FMP subscription/tier issue — a single sequential call to company.profile("AAPL") and company.shares_float("AAPL") returns valid data on the current plan.

Root cause

The MCP server hands every tool the same global singleton client (get_fmp_client()), and every one of the 22 *_tools.py categories wraps its work in async with client: — which calls start() on entry and close() on exit. Under concurrent requests the tools share one aiohttp session, so the first request to finish closed the session out from under the others:

• await session.close() tears down the connector while another request is mid-flight → Connector is closed (retried, "attempt 2/4").
• self._session = None, so the retry hits self._session.get(...) on None → 'NoneType' object has no attribute 'get'.

Reproduced deterministically: one shared client + 12 concurrent async with scopes → 11/12 failed with exactly these errors.

Fix

Reference-count the session lifecycle in FMPBaseClient.start()/close() under an asyncio.Lock: overlapping scopes share a single session, and it is only torn down when the last active scope exits. The reference is counted only after the session is guaranteed to exist, so a failed session creation can't leak a reference.

Because every tool funnels through FMPBaseClient.start/close, this one change fixes all 22 tool categories — no tool code changes.

Verification

Check	Result
New regression tests (tests/test_base.py)	3 pass (failed on old code first)
Full suite	931 passed
ruff check	clean
Live concurrency repro (direct client)	12/12 ok (was 11/12 failing)
Live MCP server, 12 concurrent get_shares_float/get_company_profile	12/12 success
Raw container logs (rebuilt image)	175 tool calls, 0 "Connector is closed", 0 "NoneType", 0 ERROR/WARNING

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