fix(provider): duplicated Bedrock/SSE fixes in provider.ts's legacy custom loader missed by earlier ports #242

Description

This fork has some provider logic duplicated in two places: a canonical copy in a packages/core plugin/helper, and a legacy copy inside packages/opencode/src/provider/provider.ts's "custom" loader / its own wrapSSE(). Two upstream (anomalyco/opencode) bug fixes were ported to the canonical copy but missed the duplicate:

1. Bedrock model-id resolution (amazon-bedrock custom loader's getModel()): double-prefixes full Bedrock ARNs (arn:..., which Bedrock rejects) and over-matches "deepseek" (prefixing non-R1 DeepSeek models that don't need/support cross-region inference). Already fixed in packages/core/src/plugin/provider/amazon-bedrock.ts (commit d66cefea74, upstream ac1758c0e6), not in provider/provider.ts's own copy.
2. wrapSSE() reader-cancel rejection: the timeout path calls void reader.cancel(err), leaving an unhandled promise rejection when the stream already errored. Already fixed in packages/core/src/aisdk.ts (commit ac3b15ba8d, upstream 69c172e8a7), not in provider/provider.ts's own wrapSSE().

How to verify

• tsgo --noEmit on packages/core and packages/opencode.
• oxlint on the changed file.
• Re-read the fix against the sibling file it duplicates to confirm it's the same change.