fix: preserve user-defined keys nested below preserved tiers

[aicayzer]

Fixes #93, fixes #94.

Problem

The PRESERVE_CHILD_KEYS preservation in toCamelCaseKeys()/toSnakeCaseKeys() is exactly one level deep. Two real bug families live one level lower:

• dynamic_variables.dynamic_variable_placeholders.<name>: the wrapper key stays snake_case, the SDK request serializer (unrecognizedObjectKeys: "strip") doesn't recognise it and deletes the whole field, so every placeholder change pushed via the CLI is a silent no-op (agents push silently ignores all changes to dynamic_variable_placeholders #93)
• query_params_schema.properties.<name> and request_body_schema.properties.<name> (any depth): user-defined property names get camelCased while required keeps the original strings (user-defined keys under properties (query_params_schema, request_body_schema) are camelCased on push, leaving required[] mismatched #94)

Before this change:

{"dynamicVariables":{"dynamic_variable_placeholders":{"transactionId":"txn_1"}}}   // wrapper stripped by the SDK, names mangled
{"queryParamsSchema":{"properties":{"userId":{...}},"required":["user_id"]}}       // schema inconsistent

After:

{"dynamicVariables":{"dynamicVariablePlaceholders":{"transaction_id":"txn_1"}}}
{"queryParamsSchema":{"properties":{"user_id":{...}},"required":["user_id"]}}

Fix

• The 'names-only' mode now consults PRESERVE_CHILD_KEYS: a preserved-tier key nested directly inside another preserved tier (e.g. dynamic_variable_placeholders under dynamic_variables) is converted itself while its children are preserved. Mirrored in toSnakeCaseKeys, which fixes the pull direction too
• Added dynamic_variable_placeholders/dynamicVariablePlaceholders and properties to the set. properties children are user-defined JSON-schema property names everywhere in this API (checked every properties field in the SDK's types; all are user-keyed Records), and because normal-mode recursion re-applies the set at every level, nested request_body_schema objects are covered at any depth

Behaviour is unchanged for flat dynamic_variables maps (test configs), request_headers, language_presets, data_collection, model_usage and nodes/edges; the existing suite covers these and still passes.

Known limitation: a user-defined key that exactly matches a preserved-tier name (e.g. a data_collection item literally named properties, or a workflow node named dynamic_variables) is now treated as a nested tier inside preserved contexts. That's inherent to any name-based preservation heuristic; flagging it here for the record.

Tests

13 new tests: 7 transform-level (utils.test.ts) and 6 through the real API functions with mock clients (casing.test.ts), covering push, pull and round-trip for both families. All 6 seam tests fail on main without the fix. Full suite 215/215, lint and build clean.

Note: built on main independently of #92 (which adds path_params_schema to the same set); happy to rebase whichever lands second.