fix: guard add_tool_field against an orphaned tool_id

[AmirF194]

Root cause

ToolInstance.tool_id (backend/database/db_models.py) has no foreign key on
ToolInfo.tool_id, so a ToolInstance row can outlive the ToolInfo row it
points to. add_tool_field does not account for that: query.first() can
return None, and the very next line dereferences tool.params, which is
exactly the traceback in #3650:

File backend/database/tool_db.py, line 442, in search_tools_for_sub_agent
AttributeError: 'NoneType' object has no attribute 'params'

The issue's own suggested fix, tool_params = tool.params or [], does not
fix this: the crash happens evaluating tool.params when tool itself is
None, one step before that fallback would ever run. I confirmed this by
reproducing both shapes in isolation: a None tool raises the reported
AttributeError, while a tool.params of None raises a different
TypeError on the iteration below it.

Fix

• add_tool_field returns None when no ToolInfo row matches the given
  tool_id, instead of crashing.
• Its caller, search_tools_for_sub_agent, skips a None result so one
  orphaned tool reference no longer aborts the whole agent run.
• tool.params also defaults to [], covering a ToolInfo row that
  legitimately has no configured params (the column is nullable), which
  matches the issue title.

Verification

• New regression tests in test/backend/database/test_tool_db.py
  (test_add_tool_field_missing_tool_info_returns_none,
  test_add_tool_field_null_params_defaults_to_empty_list,
  test_search_tools_for_sub_agent_skips_orphaned_tool) fail on unmodified
  develop with the exact reported AttributeError and pass on this branch.
• Full test/backend/database and test/backend/agents suites (1670 tests,
  includes test_create_agent_info.py, which exercises the calling path)
  pass via test/run_all_test.py in the CI image (python:3.11,
  uv sync --extra test, uv pip install -e "../sdk[dev]").
• Coverage confirms both new branches are exercised (95% on tool_db.py,
  no changed line in the miss list).
• Not checked: an actual Postgres row with tool_id missing end to end;
  the mock reproduces the exact code path from the traceback instead.

Fixes #3650

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