fix: preserve exception type when a transport error has a blank message

[acrogenesis]

Problem

When a request raised a transport exception, the high-level helpers caught it with:

errors = [{"message": str(e)}]

But httpx timeout and connection-reset exceptions carry an empty message — str(httpx.ReadTimeout("")) is "". So a timed-out (or reset) request surfaced as:

[{"message": ""}]

A blank, undiagnosable error: the exception type is lost, so callers can't tell a timeout from any other failure. This is most visible on async_mutate, where async httpx timeouts always stringify to "" (the sync path happens to carry "timed out").

Fix

Add an exception_errors helper that falls back to repr(error) when the message is blank, preserving the exception class name (e.g. ReadTimeout('')). A non-empty message is still reported verbatim. Wired into query, mutate, async_query, and async_mutate.

def exception_errors(error: Exception) -> list[dict]:
    return [{"message": str(error) or repr(error)}]

Tests

New integration test (tests/pygqlc/gql_client/test_timeout_empty_message.py) drives a real GraphQLClient against a local HTTP server stalled past post_timeout (no mocks), asserting that both sync and async paths surface a non-blank, timeout-identifying error. Plus unit coverage that a blank-message exception keeps its type and a real message is preserved verbatim. All 4 pass; each fails against the pre-fix code.

No new lint findings introduced (pre-existing ruff findings in GraphQLClient.py left untouched).

🤖 Generated with Claude Code

[palantir-valiot]

Overall Assessment

This PR introduces an exception_errors helper that falls back to repr(error) when str(error) is blank, preventing transport exceptions (e.g., httpx.ReadTimeout('')) from surfacing as [{"message": ""}]. The change is applied consistently to query, mutate, async_query, and async_mutate. Version bump, changelog entry, and new tests (real-server integration + unit) all follow repo conventions. No blocking bugs found.

Findings

No actionable findings.

Notes

• The docstring on exception_errors (lines 100-108 of GraphQLClient.py) explicitly documents the design rationale, so the str or repr fallback is intentional rather than an oversight.
• Test coverage is strong: four new tests (two unit, two integration) each fail against the pre-fix code; the integration tests drive a real GraphQLClient against a stalled local HTTP server with no mocks, aligning with AGENTS.md guidance.
• No remaining instances of the old [{"message": str(e)}] pattern exist in the codebase after this change.