Root object schemas ignore const and enum constraints

Finding: An external report on PrefectHQ/fastmcp issue #5657 (https://github.com/PrefectHQ/fastmcp/issues/5657) says that, on main at commit e2fcc412001ff7414095c5dc282f94c70299a612 (Python 3.12.13, Pydantic 2.13.4, MCP 2.0.0, macOS arm64), json_schema_to_type_adapter does not enforce const or enum when the root schema also declares type object. Per the report, an object outside the allowed value passes validation, whereas the same literal constraint without type rejects it. Reported reproduction (paraphrased from the issue): build an adapter from a schema of the form {type: object} combined with either a const of {mode: safe} or an enum containing that single object, then validate {mode: other}. The reporter states both variants are accepted unexpectedly, although both should raise a pydantic ValidationError. No exact error message exists because the failure is a missing rejection rather than an exception. The reporter also says the problem affects the adapter used to parse structured tool results. In practice, this could mean a structured result that violates a root-level literal constraint might not be rejected by that parsing path. This impact is stated by the reporter and was not independently checked here. Status: the supplied data shows the issue open, with no maintainer comments, no confirmed root cause, no documented workaround, and no linked fix commit or release. The cause is therefore unknown from these sources. Because the issue states the same constraint without type is rejected, the type-object handling path is a plausible area to inspect, but that is a hypothesis rather than confirmed. Limitations: this is a single external report (https://github.com/PrefectHQ/fastmcp/issues/5657) and was not reproduced by KFA; there is no KFA reproduction. Comment coverage is partial and later discussion may exist. Affected versions beyond the stated main commit are unknown. Users who rely on root-level const or enum with type object for validating tool outputs should treat enforcement as unverified until maintainers respond or a fix is released.