feat(mcp): move MCP implementation to strands.mcp, deprecate old path (2/3)

[fede-kamel]

Strategy: 3-PR stack for a large refactor

Issue #1431 asks for two things at once: relocating the MCP package AND preserving backwards compatibility. An atomic implementation of both is ~2000 lines of changes (mostly because git can't detect a rename when both paths must keep working). Instead, the work is split into three stacked PRs:

Step	PR	What it does	Size
1 / 3	#2148	Introduce strands.mcp as a pure re-export of strands.tools.mcp. Additive only.	+42 / -0
2 / 3	#2152 (this PR)	Move implementation to strands.mcp via git mv; strands.tools.mcp becomes a sys.modules-based deprecation shim.	+38 / -28 (git -M)
3 / 3	#2158	Move tests, add backcompat-aliases suite, update tests_integ/mcp, README, and AGENTS.md.	+149 / -63

Each step leaves main in a valid state, is independently revertable, and is independently reviewable. The order matters:

• Step 1 must precede this one so that strands.mcp is part of the public API before the implementation moves.
• Step 3 must follow this one because the integration tests and docs can't reference the new canonical path until the code actually lives there.

Why sys.modules aliasing instead of per-module stub files?

A naive implementation would create 5 per-submodule stub .py files at src/strands/tools/mcp/ (each doing from strands.mcp.<name> import * and emitting a DeprecationWarning). That's semantically identical but has two costs:

1. git can't see the move as a rename. Both paths end up with a file, so git reports "modified at old path" + "new file at new path", inflating the diff to ~2000 lines for a 1200-line module crossing the boundary. label-size fails as advisory.
2. Multiple warnings per process. Each submodule emits its own DeprecationWarning.

Instead, this PR registers all legacy submodule paths in sys.modules from a single strands/tools/mcp/__init__.py:

for name in ("mcp_agent_tool", "mcp_client", "mcp_instrumentation", "mcp_tasks", "mcp_types"):
    sys.modules[f"strands.tools.mcp.{name}"] = getattr(_mcp, name)

This means:

• The legacy submodule .py files can be deleted — git detects the 5 moves as pure renames and the diff collapses to just the import-path adjustments inside the moved files.
• One DeprecationWarning per process (at first legacy import), pointing users to the new path.
• from strands.tools.mcp.mcp_client import X still resolves: Python imports strands.tools.mcp first (running the shim __init__.py, which registers the alias), then finds strands.tools.mcp.mcp_client in sys.modules.

The full ~1800 lines of implementation still physically move across the package boundary — git's rename detection just displays the move honestly as renames instead of add+delete.

---

Summary

Second of three stacked PRs for #1431. Depends on #2148.

Moves the MCP implementation files from strands.tools.mcp into strands.mcp using git-tracked renames. strands.tools.mcp becomes a deprecation alias that:

1. Re-exports the public API from strands.mcp (object identity preserved).
2. Emits a DeprecationWarning at package-import time.
3. Registers legacy submodule paths in sys.modules so from strands.tools.mcp.mcp_client import X continues to resolve.

What this PR does

• git mv for 5 submodule files (mcp_client.py, mcp_agent_tool.py, mcp_types.py, mcp_tasks.py, mcp_instrumentation.py) from src/strands/tools/mcp/ → src/strands/mcp/.
• Updates relative imports inside the moved files (3-dot → 2-dot).
• Rewrites src/strands/mcp/__init__.py to use local imports (replacing the pure re-export from feat(mcp): add strands.mcp as canonical MCP import path (1/3) #2148).
• Rewrites src/strands/tools/mcp/__init__.py as a sys.modules-aliasing deprecation shim.

Backwards-compat guarantees (verified E2E)

• from strands.tools.mcp import MCPClient → works, emits DeprecationWarning.
• strands.tools.mcp.MCPClient is strands.mcp.MCPClient → True.
• from strands.tools.mcp.mcp_client import MCPClient → works via sys.modules alias; same object as new path.
• from strands.tools.mcp import mcp_client → works via sys.modules alias.

Tests run locally (Python 3.13)

Check	Command	Result
Full unit suite	hatch test --python 3.13 tests/	2614 passed
Lint	hatch fmt --linter --check	ruff clean
Type check	mypy ./src via hatch-static-analysis	Success, 143 source files
MCP-scoped suite	hatch test tests/strands/mcp/ tests/strands/tools/	464 passed
E2E new path vs real stdio MCP server	python demo_new_path.py — spawns tests_integ/mcp/echo_server.py	0 DeprecationWarnings; echo + get_weather called
E2E legacy path vs real stdio MCP server	python demo_backcompat.py	1 DeprecationWarning; identity check passed; submodule-path imports resolve via sys.modules
E2E legacy path vs public awslabs.aws-documentation-mcp-server	python demo_aws_docs_backcompat.py (spawned via uvx)	live HTTP to proxy.search.docs.aws.com; real AWS Lambda docs returned
Agent + MCP integration (new path, non-Bedrock model via API key)	ad-hoc smoke script	✅ agent invoked echo tool, returned expected string
Agent + MCP integration (legacy path, non-Bedrock model via API key)	ad-hoc smoke script	✅ same flow, 1 DeprecationWarning at import

Refs #1431