fix: strip <think> tags from modal processor fallback responses (closes #159)

[jwchmodx]

Problem

Reasoning models such as DeepSeek-R1 and Qwen2.5-think prepend their chain-of-thought inside <think>…</think> blocks before emitting the final answer.

When _robust_json_parse cannot extract a valid JSON object (e.g. because the model emitted only thinking content without a trailing structured answer), the four modal-processor parse methods fell back to returning the raw LLM response string as the enhanced_caption and summary. This caused internal model reasoning to be stored in the knowledge graph instead of the actual content description — exactly the symptom reported in #159.

Affected methods (all shared the same pattern):

• ImageModalProcessor._parse_response
• TableModalProcessor._parse_table_response
• EquationModalProcessor._parse_equation_response
• GenericModalProcessor._parse_generic_response

Fix

Added a static helper BaseModalProcessor._strip_thinking_tags(text) that removes <think>…</think> and <thinking>…</thinking> blocks (case-insensitive, handles multiline content) and applies it in the except fallback branch of all four methods before the cleaned text is stored or returned.

# Before
return response, fallback_entity          # raw think-tag content stored

# After
cleaned = self._strip_thinking_tags(response)
...
return cleaned, fallback_entity           # only final-answer text stored

The helper is intentionally a @staticmethod on the base class so subclasses and future processors can reuse it without extra imports.

Tests

tests/test_strip_thinking_tags.py — 13 new unit tests:

• Tag variants (<think>, <thinking>, uppercase)
• Multiline and multiple blocks
• No-op on plain text / empty string
• Full fallback path exercised for all four processor classes

All 13 pass; ruff check + ruff format --check clean.

Checklist

• Bug reproduced and root cause identified
• Fix applied to all four affected parse methods
• Unit tests added (13 tests, all passing)
• ruff lint + format checks pass
• No breaking changes to public API