feat(py/plugins/amazon-bedrock): finish the sample app and README

[hilariie]

Why

This is slice 8 of #5820, stacked on #6046

Changes

The sample gains five flows, matching the Go repo's example coverage: describe_image, summarize_pdf, prompt_caching, generate_image, and rerank. Rerank is a method on the plugin instance (Python core has no reranker primitive), so the sample now keeps a reference to the Bedrock it passes to Genkit, the pattern the plugin README documents. The image and PDF inputs are inline constants (a 64x48 striped PNG, a 722-byte PDF). summarize_pdf runs on Claude since document support is per model, and Claude is what the Go plugin's own document example uses.

Three env-gated live tests cover the paths that had none: Converse image input, document input, and a cache read.

The plugin README gains AWS setup (model access, IAM, credentials), a plugin options table, inference profiles, prompt caching, and troubleshooting. The IAM policy adds the inference-profile resource the Go README omits; without it
every us model ID fails with AccessDeniedException. Troubleshooting also documents, verified live, that a model missing from the calling region fails as "ValidationException: The provided model identifier is invalid" on both Converse and InvokeModel, not as ResourceNotFoundException.

The sample README covers the new flows and now suggests us-west-2, as it is the only region with active text-to-image models, it has both rerank models, and it runs everything here except the us-east-1-only Nova 2 embedder. The sample
also gets its missing row in samples/README.md.

Dev UI Screenshots

[gemini-code-assist]

Code Review

This pull request expands the Amazon Bedrock plugin for Genkit Python by adding comprehensive documentation, live tests, and sample flows for advanced features including vision/document input, prompt caching, image generation, and reranking. The review feedback focuses on improving robustness: specifically, using the standard response.text property in tests to avoid potential AttributeErrors when accessing message content, and safely handling potential None values during text part concatenation in the sample application to prevent TypeErrors.

[cabljac]

Docs check out against the code. Options table, inference-profile prefixes, embedder table and the cache-token behaviour all match what the plugin actually does, and the sample's flows line up with the core API. One thing inline.

[cabljac]

Approving. The quick-start model ID is worth fixing before this merges, since it's the first thing a new user copies, but it's a two-line change and nothing else needs to block on it.

[hilariie]

• Quick start now uses us.anthropic.claude-sonnet-4-5-20250929-v1:0
• The rebase surfaced a stale Plugin options table. It listed the old hardcoded client defaults and was missing total_timeout. Those knobs are all unset now and fall back under your AWS config, so the table says that instead.
• Dropped the leftover Go comparisons from the docs and comments.