Add null filters and bounded Boolean filter groups

[vara-bonthu]

Problem

The semantic query API combines all request filters with AND. It also does not support common null operations.

The API cannot represent questions such as:

• Which accounts have no assigned customer-success manager?
• Which contracts have no renewal date?
• Which accounts are enterprise customers or are located in EMEA?
• Which tickets are escalated or outside SLA?

Proposed solution

Extend the request filter model with:

• IS NULL
• IS NOT NULL
• NOT LIKE
• Explicit AND groups
• Explicit OR groups
• A bounded nesting depth
• A bounded total predicate count

Example:

{
  "filter": {
    "or": [
      {
        "field": "account.segment",
        "op": "=",
        "value": "Enterprise"
      },
      {
        "field": "account.region",
        "op": "=",
        "value": "EMEA"
      }
    ]
  }
}

All referenced fields must still resolve through the compiled model and pass governance checks.
The request must never accept raw SQL predicates.

Implementation areas

• internal/planner/plan.go
• internal/planner/plan_sql.go
• internal/serving/limits.go
• internal/serving/mcp/mcp.go
• REST and planner tests

Acceptance criteria

• IS NULL and IS NOT NULL are supported.
• NOT LIKE is supported.
• Explicit AND and OR groups are supported.
• Nesting depth and total predicates are bounded.
• Every referenced field is authorized.
• Raw SQL cannot enter the filter tree.
• Request hashing includes the complete filter structure.
• StarRocks and Trino emit equivalent Boolean semantics.