fix: cap mcp below 2.0 and stop a failed mcp import breaking import any_agent

[njbrake]

Description

mcp 2.0 renamed streamablehttp_client to streamable_http_client. The dependency was uncapped (mcp>=1.5.0), so a fresh install resolves mcp 2.x and import any_agent fails with NameError: name 'MCPTool' is not defined.

Two separate fixes:

1. Cap at mcp>=1.5.0,<2. This is what unbreaks installs. Soft deprecation means no migration to mcp 2.x for now.
2. mcp_client.py records missing_mcp_error and raises a readable ImportError from model_post_init, but that guard never ran: ClientSession and MCPTool appear in annotations evaluated at class creation, so import raised NameError first. Both now get placeholders in the except ImportError branch.

Fix 2 alone is not enough. mozilla-ai-tinyagent 0.1.1 has the identical bug and is imported first via any_agent.config, so the cap is what protects users today. Reported upstream at mozilla-ai/tinyagent#22.

Verified against a real mcp 2.2.0 install: with both copies guarded, import any_agent succeeds and MCPClient(...) raises the intended ImportError.

PR Type

• Bug Fix

Relevant issues

Upstream: mozilla-ai/tinyagent#22

Checklist

• I understand the code I am submitting.
• I have added unit tests that prove my fix/feature works
• I have run this code locally and verified it fixes the issue.
• New and existing tests pass locally
• Documentation was updated where necessary
• I have read and followed the contribution guidelines
• AI Usage:
  • No AI was used.
  • AI was used for drafting/refactoring.
  • This is fully AI-generated.

Note on "existing tests pass": tests/unit gives 278 passed with 4 failures that are present on main before this change and unrelated to it (any-llm-sdk no longer has a gateway provider, plus two OpenAI-path tests).

AI Usage Information

• AI Model used: Claude Opus 5

• AI Developer Tool used: Claude Code

• Any other info you'd like to share: Written under @njbrake's direction. The reasoning and decisions are his.

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[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tests/unit/tools/mcp/test_mcp_import_guard.py	97.91%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
src/any_agent/tools/mcp/mcp_client.py	77.94% <100.00%> (-3.41%)	⬇️
tests/unit/tools/mcp/test_mcp_import_guard.py	97.91% <97.91%> (ø)

... and 42 files with indirect coverage changes

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