feat(workflow): support nested dot-notation in webhook trigger fields (#4235)

[iroiro147]

What

Webhook-triggered workflows received only top-level JSON fields because nested objects were stringified into a single value. A CloudEvents-style body like

{"type":"workflow.failed","data":{"title":"Deploy","reason":"Timeout"}}

surfaced {{trigger.type}} → "workflow.failed" but {{trigger.data.title}} was unreachable — data itself was the entire serialized JSON, unusable as a sub-template key. Senders had to duplicate useful fields at the top level just for Buzz.

Both webhook-population sites had the same flat-stringify pattern (bridge.rs /workflows HTTP endpoint, command_executor.rs message_posted trigger with JSON content). This PR introduces a shared flatten_webhook_fields helper in buzz-workflow::executor that recursively flattens nested JSON objects/arrays into dot-notation keys, and swaps both call sites to use it.

Behavior

Template	Before	After
{{trigger.data.title}}	unresolved literal	"Deploy"
{{trigger.data}}	serialized JSON string	serialized JSON string (unchanged)
{{trigger.data.meta.attempt}}	unresolved	"3"
{{trigger.items.0.name}}	unresolved	array index lookup
{{trigger.count}} (top-level scalar)	"42"	"42" (unchanged)

Objects and arrays are still stored under their prefix as serialized JSON, so consumers who want the whole envelope continue to work. Existing top-level templates are bit-for-bit identical.

Implementation

One new shared function flatten_webhook_fields(map: &serde_json::Map<String, serde_json::Value>) -> HashMap<String, String> in crates/buzz-workflow/src/executor.rs:

• Recursively walks objects/arrays → data.title, items.0.nested
• Stores serialized JSON at the parent prefix for compatibility
• Scalar values stringified; strings kept as-is

Both former inline loops replaced with one call:

• bridge.rs:1889 (HTTP /workflows/{id}/webhook)
• command_executor.rs:919 (message_posted trigger parsing event.content as JSON)

Tests

Four new unit tests in crates/buzz-workflow/src/executor.rs:

• flatten_nested_object_produces_dotted_keys — CloudEvents-style shape
• flatten_scalar_types_convert_to_strings — number/bool/null
• flatten_array_produces_indexed_dotted_keys — items.0 etc.
• resolve_template_uses_nested_webhook_field — end-to-end substitution

cargo test -p buzz-workflow --lib — 157 passed, 0 failed (4 new + 153 existing).
cargo test -p buzz-relay --lib bridge — 59 passed, 0 failed.
cargo check -p buzz-workflow -p buzz-relay — clean.

Linked issue

Refs #4235