[hotfix][java][python] Harden Bash arithmetic evaluation

[wenjin272]

Linked issue: N/A (hotfix)

Purpose of change

Bash can reinterpret values during integer declarations and arithmetic expansion after the original command has passed AST validation. This creates execution paths that are not covered by the command allowlist.

This change applies the same fail-closed policy in Java and Python:

• reject declaration commands;
• reject arithmetic expansions;
• reject standalone variable assignments while preserving assignment prefixes attached to an allowed executable; and
• add validator and tool-level regression coverage for both indirect evaluation paths.

This intentionally makes validation stricter: benign declaration commands, arithmetic expansions, and standalone assignments are also rejected.

Tests

• pytest -q python/flink_agents/plan/tools/bash/tests/test_bash_tool.py (46 passed)
• mvn -pl plan -am -DskipITs -Dtest=BashValidatorTest,BashToolTest -Dsurefire.failIfNoSpecifiedTests=false test (21 passed)
• JAVA_HOME=/Users/jhin/Kit/jdk-17.0.2.jdk/Contents/Home mvn -pl plan -am -DskipITs test (713 tests, 0 failures/errors, 12 skipped)
• ruff check and ruff format --check for the changed Python files
• git diff --check

API

No public API changes. Bash tool validation behavior is intentionally stricter.

Documentation

• doc-needed
• doc-not-needed
• doc-included

Was this patch authored or co-authored using generative AI tooling?

• Yes
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Generated-by: Codex (GPT-5)