CLI: add stdin support for instance validation

[Vaibhav701161]

Adds stdin support for instance input to enable real-time editor diagnostics.
Fixes(#537)

• Centralized stdin parsing in src/input.h (JSON-first, YAML fallback).
• validate accepts instances from stdin only; schemas must be file paths.
• Stdin inputs resolve relative $ref against the current working directory.
• Includes integration test test/validate/pass_stdin.sh.
• Unblocks sourcemeta/studio#61; addresses sourcemeta/jsonschema#537.

Refs: sourcemeta/studio#61, #537

*Build and test:

[Vaibhav701161]

Hi @jviotti , I’ve opened this PR and would appreciate a review when you have time. Thanks!

[jviotti]

Hey @Vaibhav701161 , sorry for the delay. It has been a hectic week. I'll try to get into it today

[jviotti]

I like the idea but I think there are some things to iron out:

• If we accept standard input, we should do so in ALL commands, otherwise it gets weird
• Some commands require a schema as input but for others, passing nothing means "the current directory", which introduces some ambiguity. For example, jsonschema fmt formats all schemas in the current directory, while jsonschema inspect <schema.json> requires a single schema argument
• Furthermore, some commands require 2 schemas, like validate

I think for consistency, we can require the - to signify standard input in every command. However, for commands take potentially take directories as input, we should transparently implement this logic on src/input.h. So it happens out of the box.

The other problem here will be line numbers. We should make sure all relevant commands handle standard input while still reporting line/column information and potentially some "special" file path like <stdin>?

[Vaibhav701161]

Hey @jviotti , thanks a lot for the feedback.

I’ve pushed an updated iteration that tries to address the concerns you mentioned:

• - is now the explicit opt-in for stdin.
• The stdin logic is centralized in src/input.h, so it’s not handled ad-hoc in individual commands.
• Ambiguous cases (schema via stdin, multiple stdin inputs) are explicitly rejected.
• Line/column information is preserved, and stdin is handled as a special input (covered via tests).