fix(sdk): omit null/empty request fields so older instances accept them

[t0saki]

Description

The Python SDK's find / search / add_resource unconditionally attach optional fields to their request bodies — args as an empty {} and tags / score_threshold / filter / context_type / session_id as null — even when the caller never set them. Because OpenViking kernels declare model_config = ConfigDict(extra="forbid") on these routes, any instance that predates a given field rejects the entire request with body.<field>: Extra inputs are not permitted. This patch stops sending those empty/null fields.

This is the SDK-side counterpart of #2799, which fixed the exact same anti-pattern in the Rust CLI (crates/ov_cli/src/client.rs) but left the Python SDK (sdk/python/openviking_sdk/client.py) untouched. The SDK is what every Python consumer uses — including vikingbot, which talks to OpenViking through ov.AsyncHTTPClient and therefore hits this against older instances on all three routes.

Related Issue

Follow-up to #2799 (same breakage, different client surface).

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

• Add AsyncHTTPClient._compact_request_body() and apply it in find / search / add_resource: drop null-valued keys and an empty args object right before sending. Scoped to these read/create routes only, where a missing optional field and an explicit null are equivalent — deliberately not applied globally, since null may carry "clear this field" semantics on a future update/PATCH route. Mirrors the CLI's compact_request_body from fix(cli): omit null/empty request fields so older instances accept them #2799.
• The synchronous SyncHTTPClient delegates to the async methods, so it inherits the fix with no separate change.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • macOS

New unit tests in tests/client/test_http_client_compact.py:

• _compact_request_body drops null keys and an empty args, keeps False / non-empty objects / non-empty args.
• find / search omit tags / score_threshold / filter / context_type / session_id when unset, and keep tags when explicitly provided.
• add_resource omits an empty args and null to / parent / timeout / ignore_dirs / include / exclude, and keeps a non-empty args.

pytest tests/client/test_http_client_compact.py → 7 passed. The two pre-existing LocalClient errors in tests/client/test_search.py are unrelated (they reproduce on main with this patch stashed).

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

Additional Notes

ruff check passes on the changed files. ruff format --check flags only one pre-existing formatting drift in an unrelated method (SyncHTTPClient.delete_skill), left untouched to keep the diff scoped to this fix.