fix(bigtable): stop resolving view parameters from caller-writable state

[prasanna8585]

BigtableParameterizedViewTool wraps Bigtable's execute_sql to automatically inject values (like user_id) into a parameterized view's VIEW_PARAMETERS(), so the view itself can restrict which rows a query is allowed to see. The class's own docstring states this "securely restricts query execution to the logged-in user's data" -- but the resolution logic had an undocumented fallback: when a configured view_parameter_name did not match a strongly-typed attribute of tool_context (which user_id always does, via ReadonlyContext), it fell back to tool_context.state[param_name].

tool_context.state is writable by the caller -- the same fact this same PR's sibling fix (BigQuery/AgentEngineSandboxComputer, in query_tool.py and sandbox_computer.py) already established for a different pair of tools in this same file tree. A view parameter taken from state lets the caller choose which user's (or tenant's) rows a "securely restricted" parameterized view returns, defeating the row- level restriction the feature exists to provide. This is realistic for any application-defined scoping parameter beyond the documented user_id example -- tenant_id, org_id, customer_id, or any other custom name a multi-tenant deployment configures via view_parameter_names, none of which are built-in ToolContext/ReadonlyContext attributes.

Verified this was never previously addressed: the feature was introduced in 14a24f2 (PR #6128, "Support parameterized views with secure parameter injection") and has had no subsequent fix; the only later commit touching this file is an unrelated mypy-typing refactor.

Fix: remove the tool_context.state fallback entirely. Only names that resolve to a real tool_context attribute are honored now; anything else is silently omitted from view_parameters, exactly as an already- missing attribute was previously handled.

Verified: existing test suite (14 tests) run and pass, including the two pre-existing tests that only exercise the safe, strongly-typed path (user_id), which are unaffected. The test that asserted the vulnerable fallback behavior (test_bigtable_parameterized_view_tool_execution_session_state_fallback) is replaced with test_bigtable_parameterized_view_tool_ignores_state_for_custom_parameters, asserting the opposite: a caller-controlled tenant_id sitting in session state is not used. Confirmed this new test actually catches the bug by temporarily reverting the fix locally and re-running it -- it fails against the vulnerable code and passes against the fix. test_bigtable_parameterized_view_tool_execution_multiple_parameters is updated to match: tenant_id/agent_id (state-only) are now correctly omitted from the result, while user_id (a real attribute) still resolves.