### What problem does this PR solve?
Fixes#15542.
AWS Bedrock support for the Go model provider layer was added in #15166,
but embedding support was intentionally left out of scope and
`BedrockModel.Embed(...)` still returned the `no such method` sentinel.
This PR implements Bedrock text embeddings under the umbrella provider
tracker #14736.
### What this PR includes
- `internal/entity/models/bedrock.go`: implement
`BedrockModel.Embed(...)` through Bedrock Runtime `InvokeModel` with
existing SigV4 auth, region resolution, and runtime URL helpers.
- Titan embeddings: supports `amazon.titan-embed-text-v1` and
`amazon.titan-embed-text-v2:0`; v2 forwards `EmbeddingConfig.Dimension`
as `dimensions` when provided, while v1 keeps the payload minimal.
- Cohere embeddings: supports `cohere.embed-english-v3`,
`cohere.embed-multilingual-v3`, and `cohere.embed-v4:0`; batches input
texts and maps returned vectors to RAGFlow `EmbeddingData` in input
order.
- `conf/models/bedrock.json`: adds the `embedding` URL suffix (`invoke`)
and Bedrock embedding model entries.
- `internal/entity/models/bedrock_test.go`: adds unit tests for Titan,
Cohere, typed Cohere responses, validation, empty input, unsupported
models, and HTTP error propagation.
Reference docs:
- Bedrock InvokeModel API:
https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_InvokeModel.html
- Titan Text Embeddings:
https://docs.aws.amazon.com/bedrock/latest/userguide/titan-embedding-models.html
- Cohere Embed models on Bedrock:
https://docs.aws.amazon.com/bedrock/latest/userguide/model-parameters-embed.html
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### How was this tested?
- [x] `jq empty conf/models/bedrock.json`
- [x] `git diff --check`
- [x] `go test ./internal/entity/models/... -run Bedrock -count=1`
- [x] `go test ./internal/entity/models/... -run '^$' -count=1`
- [x] `go test ./internal/entity/models/... -run Bedrock -race -count=1`
Note: `go test ./internal/entity/models/... -count=1` currently fails in
unrelated existing Astraflow coverage
(`TestAstraflowEmbedReturnsNoSuchMethod` panics in
`internal/entity/models/astraflow.go`). The Bedrock-specific tests and
compile-only package check pass.
## Summary
Closes#15165.
Implements the AWS Bedrock model provider for the Go API server, tracked
under #14736. Adds Converse + Converse-Stream chat and foundation-model
listing, with SigV4 signing over a hand-rolled `net/http` path that
matches the established pattern in `internal/entity/models/` (no new
direct `go.mod` deps).
## Linked tracker
Tracked under #14736 (Implement model providers of RAGFlow API server in
Go). Closes#15165.
