Process a single document synchronously with a Google Cloud Document AI processor, sending inline base64 or a GCS URI and parsing the Document object via text anchors

Documented steps

1. Create or identify a processor in the target region (Enterprise Document OCR, Layout Parser, Invoice Parser and similar) and note its resource name projects/{PROJECT_ID}/locations/{LOCATION}/processors/{PROCESSOR_ID}.
2. Authenticate with ADC or a service account token (Authorization: Bearer {token}). The identity needs documentai.processors.processOnline, granted by roles/documentai.apiUser or higher.
3. Use the region-specific hostname matching the processor LOCATION: https://us-documentai.googleapis.com or https://eu-documentai.googleapis.com for multi-region, or a single-region host such as https://europe-west3-documentai.googleapis.com. A mismatched host and location returns an error.
4. POST https://{LOCATION}-documentai.googleapis.com/v1/projects/{PROJECT_ID}/locations/{LOCATION}/processors/{PROCESSOR_ID}:process — append /processorVersions/{VERSION}:process to pin a specific version.
5. Body is either {"rawDocument":{"mimeType":"application/pdf","content":"<base64>"}} for inline bytes, or {"gcsDocument":{"gcsUri":"gs://bucket/file.pdf","mimeType":"application/pdf"}} for a Cloud Storage source.
6. Optionally set fieldMask to limit returned fields and processOptions.individualPageSelector.pages to process a page subset.
7. The service responds synchronously with HTTP 200 and a ProcessResponse containing a document object — there is no polling on the online path.
8. Parse document.text (the single source of truth for all extracted text), document.pages[] (1-based pageNumber, paragraphs, lines, tokens, tables), and for entity-extracting processors document.entities[] (type, confidence, textAnchor, normalizedValue).
9. Resolve any element's text using its textAnchor.textSegments[].{startIndex,endIndex} as offsets into document.text. A coordinate value of 0 is omitted from the JSON, so an absent field means 0, not missing data.
10. Field semantics reference: https://docs.cloud.google.com/document-ai/docs/handle-response