fix(workflow): exclude workflow tool from task agent inheritance to prevent recursion

[TAKEDA-Takashi]

Summary

• Exclude workflow tool from task agent tool inheritance in _create_task_agent() to prevent infinite recursion
• Applies to both explicit tool list specification and inherit-all-from-parent paths
• Follows the same recursion prevention pattern established by the think tool (PR fix(think): implement automatic recursion prevention for think tool #167)

Closes #383

Changes

src/strands_tools/workflow.py

• Added workflow exclusion guard in the explicit tool list loop (with WARNING log)
• Changed inherit-all path from one-liner to loop with workflow exclusion (with DEBUG log)

tests/test_workflow.py

• test_create_task_agent_excludes_workflow_tool_explicit: verifies tools: ["calculator", "workflow"] results in only calculator
• test_create_task_agent_excludes_workflow_tool_inherit_all: verifies auto-exclusion when inheriting all tools
• test_create_task_agent_only_workflow_tool_requested: verifies tools: ["workflow"] results in empty list
• test_create_task_agent_without_workflow_in_registry: regression test when parent has no workflow tool

Relationship to PR #182

This PR is complementary to PR #182 (tools: [] fix). PR #182 fixes the case where an empty tools list is specified; this PR prevents recursion when workflow appears in or is inherited into the tools list. The code changes are in different sections and can coexist.

Test plan

• hatch fmt --formatter passes
• hatch fmt --linter passes
• hatch test tests/test_workflow.py -v — 36 passed
• hatch test — 1118 passed, 25 skipped, 0 failed