fix: support dot-notation in resolveParamValue for integration step references

[Copilot]

resolveParamValue claimed to support ${step1.value} references to prior step fields, but results stores step output keyed by step name (e.g. results["step1"] = map[string]any{...}), so ${step1.value} would never resolve — only an exact key "step1.value" in results would match, which never exists.

Changes

• handlers/integration.go: After the fast exact-match path, split the variable name on . and traverse into the nested map[string]any step result. Falls back to the original string if the step is missing, the result is not a map, or the nested key doesn't exist.

// ${step1.value} now resolves correctly:
results := map[string]any{
    "step1": map[string]any{"value": "resolved"},
}
resolveParamValue("${step1.value}", results) // → "resolved"
resolveParamValue("${step1}", results)       // → map[string]any{"value": "resolved"} (exact match, unchanged)

• handlers/integration_handler_test.go: Added tests covering dot-notation success, missing nested key, non-map step result, missing step, exact match, plain value passthrough, and non-reference strings.

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