fix(providers/anthropic): serialize Bedrock system as string

[ibetitsmike]

Mux working on behalf of Mike.

Problem

For Bedrock, fantasy was serializing system as an Anthropic-style array of text blocks:

"system":[{"text":"you are helpful","type":"text"}]

Bedrock's Anthropic Messages schema expects system as a plain string, so this triggers a 400 validationException on affected paths.

Fix

Bedrock-only, post-serialization override:

• prepareParams still produces the usual Anthropic params. When useBedrock is set, it now also returns a flattened system string and leaves params.System nil.
• buildRequestOptions accepts the override and applies it via option.WithJSONSet("system", flattened).
• Both Generate and Stream go through the same seam, so the wire shape cannot drift between streaming and non-streaming.
• New helper flattenSystemForBedrock in providers/anthropic/bedrock.go joins non-empty text blocks with \n\n, preserving the existing toPrompt / groupIntoBlocks semantics.

Direct Anthropic behavior is unchanged and locked in by a new assertion.

Tests

TestBedrockSystemPromptWireShape covers Generate and Stream across:

• single system message
• three consecutive system messages joined with \n\n in order
• system, user, system preserving only the first contiguous system block (matches today's toPrompt behavior)
• direct Anthropic still sends system as an array

Recorded shapes:

• Bedrock, before: {"system":[{"text":"you are helpful","type":"text"}]}
• Bedrock, after: {"system":"you are helpful"}
• Direct Anthropic, before and after: {"system":[{"text":"you are helpful","type":"text"}]}

Scope

No changes to groupIntoBlocks, toPrompt, or any non-Bedrock path. No changes to providers/bedrock/*.