schema: ConcatAgenticMessages loses streamed MCP tool result chunks and corrupts reasoning signatures

Summary

Two field-merge defects in ConcatAgenticMessages (schema/agentic_message.go) cause data loss / data corruption when concatenating streamed agentic messages:

1. concatMCPToolResults keeps only the last content chunk

schema/agentic_message.go:1821 does ret.Content = r.Content, overwriting previous chunks. A streamed MCP tool result therefore keeps only its final chunk. The sibling concatMCPToolCalls correctly accumulates (ret.Arguments += c.Arguments, line 1781), so the asymmetry looks like a copy-paste mistake — and the existing test even asserted the buggy behavior.

msgs := []*AgenticMessage{
    {Role: AgenticRoleTypeAssistant, ContentBlocks: []*ContentBlock{
        NewContentBlockChunk(&MCPToolResult{CallID: "c1", Name: "tool", Content: `{"part1":`}, &StreamingMeta{Index: 0})}},
    {Role: AgenticRoleTypeAssistant, ContentBlocks: []*ContentBlock{
        NewContentBlockChunk(&MCPToolResult{CallID: "c1", Name: "tool", Content: `"value"}`}, &StreamingMeta{Index: 0})}},
}
m, _ := ConcatAgenticMessages(msgs)
// got:  Content == `"value"}`   (first chunk lost)
// want: Content == `{"part1":"value"}`

2. concatReasoning corrupts the signature by string-concatenating it

schema/agentic_message.go:1327 does ret.Signature += r.Signature. A reasoning Signature is an opaque integrity/encryption token, not appendable text. Adapters commonly repeat the same signature across chunks; concatenating produces e.g. sig-abcsig-abc, which fails verification when the message is sent back to the model. The old *Message API (mergeReasoningParts, schema/message.go:1491-1493) keeps the last non-empty signature — the two APIs now disagree.

Expected behavior

• MCP tool result Content chunks append (like MCPToolCall.Arguments).
• Reasoning Signature keeps the last non-empty value (parity with mergeReasoningParts).

Both reproduce on current main (9d983b36) and alpha/10. I have a fix with regression tests ready and will open a PR shortly.