Add type-driven schema generation for 1.6

[quinnj]

Summary

• restore JSONSchema 1.6 as an additive release with JSONSchema.schema(::Type) generation while keeping schema unexported
• keep Schema non-parametric and add JSONSchema.spec(schema), indexing helpers, and JSON serialization support
• cover Agentif-style tool schemas, nullable unions, nested additionalProperties, tuple variants, and draft 2020-12 tuple output
• update copyright notices and README language around the generator's draft-v7-compatible subset

Testing

• JULIA_PKG_PRECOMPILE_AUTO=0 julia --project=. -e 'using Pkg; Pkg.test()'
• temp-copy LLMProviders tests against the pushed JSONSchema branch
• JuliaFormatter no-write check via a temporary formatter environment

Co-authored by Codex

[odow]

I'm happy if you're happy

[quinnj]

Thanks. I've recently been adding "trim compile" test workloads to test suites, so wondering if I should try and do that for JSONSchema as well, but that can be a separate, follow up work. I'm also not sure my downstream packages relying on schema generation will even be trim-compile-able, so mabye not urgent anyway.

[odow]

I'd assume it's not trimmable because of this dispatch:

JSONSchema.jl/src/validation.jl

Lines 83 to 85 in 4076c12

	function _validate_entry(x, schema::AbstractDict, path)
	for (k, v) in schema
	ret = _validate(x, schema, Val{Symbol(k)}(), v, path)

Rather than some sort of enum-based type-stable but runtime lookup, I dispatch on ::Val.