Use JSON Schema for allowlist/denylist body constraints (jsonschema v6.0.2)

[winhowes]

Motivation

• Body filters previously used ad-hoc recursive subset matching which couldn't express patterns, numeric ranges or string-length constraints.
• Move to a standard schema format so capabilities and operators can describe richer, well-specified rules.

Description

• Add app/body_schema.go and wire JSON Schema validation helpers (compileBodySchema, validateBodySchema, validateBodySchemaDefinition, decodeJSONBody, formValuesToJSON) into allowlist and denylist checks (validateRequestReason, constraintMatchesRequest, validation paths).
• Switch the JSON Schema library to github.com/santhosh-tekuri/jsonschema/v6 at v6.0.2 and configure the compiler to use Draft‑07 by default via compiler.DefaultDraft(jsonschema.Draft7).
• Update capability generators (Slack, SendGrid) to emit JSON Schema bodies instead of ad-hoc value maps and update tests to assert the new schema shape.
• Update docs, examples and validation to describe that methods.<name>.body is a JSON Schema (draft‑07); remove retired ad-hoc form/recursive match helpers.

Testing

• Ran go test ./... and the package test suite completed successfully with all modified unit tests passing.

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