refactor(exceptions): dedupe src/exceptions via core re-export (#4785)

src/exceptions.py was a byte-for-byte duplicate of the canonical
core/exceptions.py. Replace its class bodies with a re-export shim
(mirroring the core/constants.py -> src/constants.py pattern) so the
exception classes are defined in exactly one place. Also fix the stale
"# src/exceptions.py" header comment in core/exceptions.py.

No behavior change: both import paths resolve to the same class objects
(verified by identity), so `except SessionNotFoundError` works regardless
of which module it was imported from. Ran py_compile and
pytest tests/test_app.py (12 passed).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/exceptions.py

@@ -1,4 +1,4 @@
-# src/exceptions.py
+# core/exceptions.py
 """Custom exceptions for the application."""
 
 class SessionNotFoundError(Exception):

src/exceptions.py

@@ -1,29 +1,22 @@
 # src/exceptions.py
-"""Custom exceptions for the application."""
+"""Backward-compatible shim — the single source of truth is core/exceptions.py.
 
-class SessionNotFoundError(Exception):
-    """Raised when a requested session is not found."""
-    def __init__(self, session_id: str):
-        self.session_id = session_id
-        super().__init__(f"Session '{session_id}' not found")
+Historically this module was a byte-for-byte duplicate of core/exceptions.py,
+which is the canonical definition (imported by app.py, core/__init__.py, and
+routes/chat_routes.py). To kill the drift, this now simply re-exports the
+exception classes from core.exceptions so there is exactly one place that
+defines them. Existing `from src.exceptions import ...` callers keep working.
+"""
+from core.exceptions import (  # noqa: F401
+    SessionNotFoundError,
+    InvalidFileUploadError,
+    LLMServiceError,
+    WebSearchError,
+)
 
-class InvalidFileUploadError(Exception):
-    """Raised when a file upload fails validation."""
-    def __init__(self, message: str, filename: str = None):
-        self.filename = filename
-        self.message = message
-        super().__init__(message)
-
-class LLMServiceError(Exception):
-    """Raised when there is an error communicating with the LLM service."""
-    def __init__(self, message: str, endpoint: str = None):
-        self.endpoint = endpoint
-        self.message = message
-        super().__init__(message)
-
-class WebSearchError(Exception):
-    """Raised when there is an error with web search functionality."""
-    def __init__(self, message: str, query: str = None):
-        self.query = query
-        self.message = message
-        super().__init__(message)
+__all__ = [
+    "SessionNotFoundError",
+    "InvalidFileUploadError",
+    "LLMServiceError",
+    "WebSearchError",
+]