Improve catalog precision for mapped active contexts

[samzong]

Problem

Catalog schema v17 exposes active-context bindings for nested runtime-schema operations and workflow steps. It currently reports every context-bound parameter, including parameters that an explicit mapping supplies. This is conservative but can make agents inspect an active context that execution does not use.

Related PR review findings:

• feat(runtime): add account-scoped active contexts #144 (comment)
• feat(runtime): add account-scoped active contexts #144 (comment)

Runtime nuance

A blanket filter by mapping key is incorrect for runtime-schema references: when a mapping references an optional target parameter that is absent, runtimeSchemaInput leaves the source parameter unchanged and active-context fallback still applies. Workflow mappings are strict and either produce a value or fail before context resolution. Literal runtime-schema mappings always suppress context fallback.

Acceptance criteria

• Workflow catalog contexts omit operation parameters supplied by step.Params.
• Runtime-schema catalog contexts omit only mappings guaranteed to produce a value, while retaining possible fallback contexts for absent optional target references.
• Catalog output remains schema-version compatible with the active-context contract.
• Focused catalog tests cover literal mappings, required target references, and optional absent target references.

This is non-blocking for PR #144: current metadata over-reports possible context use but does not change request execution or persisted state.