bug: prevent CommandSchemaRegistry lookup from mutating schemas

[codeforester]

Goal

Keep an isolated CommandSchemaRegistry schema immutable after registration unless the registry API explicitly performs a validated change.

Background

Registration normalizes and copies the supplied field mapping and rejects duplicate registration so a process cannot silently change an established record type. However, CommandSchemaRegistry.schema() returns the stored mutable dictionary itself. Any consumer with read access can add/remove fields and bypass name/type/duplicate validation.

The module-level RECORD_SCHEMAS alias is intentionally mutable for compatibility; the newer isolated registry is the recommended boundary and should not inherit that escape hatch accidentally.

Reproduction and evidence

Framework reference: 263d893 on main.

registry = CommandSchemaRegistry()
registry.register("demo", {"name": STRING})
registry.schema("demo")["enabled"] = BOOLEAN
print(registry.schema("demo"))
# {'name': STRING, 'enabled': BOOLEAN}

No register() validation or duplicate check runs for the mutation.

Scope

• Return an immutable view or defensive copy from the isolated registry lookup.
• Keep codec encode/decode behavior deterministic after registration.
• Preserve the explicitly documented module-level compatibility surface as needed.
• Update public typing/docs and add mutation-regression tests.

Acceptance criteria

• CommandSchemaRegistry.schema() cannot mutate stored registry state.
• Caller mutation of the original mapping after register() remains isolated.
• The only supported schema mutation path enforces validation and duplicate rules.
• Default compatibility helpers continue to behave as documented.

Project fields

• Status: Backlog
• Priority: P2
• Area: Python
• Initiative: v1.0 Readiness
• Size: S

Ownership

• Assignee: @codeforester