mcp: propertly validate against JSON, independent of Go values

[findleyr]

Our validation logic was avoiding double-unmarshalling as much as
possible, by parsing before validation and validating the Go type.

This only works if the Go type has the same structure as its JSON
representation, which may not be the case in the presence of types with
custom MarshalJSON or UnmarshalJSON methods (such as time.Time).

But even if the Go type doesn't use any custom marshalling, validation
is broken, because we can't differentiate zero values from missing
values.

Bite the bullet and use double-unmarshalling for both input and output
schemas. Coincidentally, this fixes three bugs:

• We were accepting case-insensitive JSON keys, since we parsed first,
  even though they should have been rejected. A number of tests were
  wrong.
• Defaults were overriding present-yet-zero values, as noted in an
  incorrect test case.
• When "arguments" was missing, validation wasn't performed, no defaults
  were applied, and unmarshalling failed even if all properties were
  optional.

First unmarshalling to map[string]any allows us to fix all these bugs.
Unfortunately, it means a 3x increase in the number of reflection
operations (we need to unmarshal, apply defaults and validate,
re-marshal with the defaults, and then unmarshal into the Go type).
However, this is not likely to be a significant overhead, and we can
always optimize in the future.

Update github.com/google/jsonschema-go to pick up necessary improvements
supporting this change.

Additionally, fix the error codes for invalid tool parameters, to be
consistent with other SDKs (Invalid Params: -32602).

Fixes #447
Fixes #449
Updates #450