fix(sdk/python/core): raise clear KeeperError for malformed InMemoryKeyValueStorage config (KSM-299)

[mgallego-keeper]

Summary

InMemoryKeyValueStorage raised a cryptic TypeError: object of type 'NoneType' has no len() when constructed with a config string that is not valid JSON or base64-encoded JSON (e.g. InMemoryKeyValueStorage("A")). This fixes KSM-299 so a malformed config produces a clear, actionable KeeperError.

Root cause

In storage.py the string-config branch reassigns config to the result of json_to_dict(config) (which is None for invalid JSON), then builds the error with ... % str(len(config)). Python evaluates the len(config) operand before constructing the KeeperError, so len(None) raises TypeError first — the intended "Could not load config data" message never surfaces.

Fix

• Keep the original config string in config_str and report its length, so the error path no longer calls len() on the parsed (possibly None) result.
• The message now explains the problem: Could not load config data. The configuration string is not valid JSON or base64-encoded JSON (text length: N).

Tests

Added two regression tests to config_error_test.py, following the existing "must raise KeeperError, not TypeError" pattern:

• single-character config ("A") — the exact ticket repro
• a non-JSON, non-base64 string

Verified locally: full config_error_test.py passes (17 tests) and the version smoke test passes.

Release / version

• Bumps python-core 17.3.0 → 17.4.0 across the three coupled locations (_version.py, the keeper_globals.py hardcoded fallback, and the smoke_test.py assertions).
• Adds a consolidated ### 17.4.0 README changelog covering KSM-299 and the other fixes staged on this release branch (KSM-1019, KSM-1080, KSM-1085).

Notes for reviewers

• Base branch release/sdk/python/core/v17.4.0 already has the latest master merged in (keeper_secrets_manager_storages v1.1.0).
• The changelog lists KSM-1019 even though its code lands via fix(sdk/python/core): honor kms_cache_file_name override in KSMCache cache path (KSM-1019) #1046 (being re-targeted onto this release branch). This PR owns the single 17.4.0 version bump and the consolidated changelog, so fix(sdk/python/core): honor kms_cache_file_name override in KSMCache cache path (KSM-1019) #1046's own version-bump/changelog content is dropped there to avoid conflicts.