fix(foundation): collapse nested if guards in parse_gemini_sse (clippy::collapsible_if #1295)

[SH20RAJ]

Summary

Fixes #1295.

Running `cargo clippy --workspace --all-features -- -D warnings` on
`main` produces three `clippy::collapsible_if` warnings in
`crates/mofa-foundation/src/llm/google.rs` inside the end-of-stream
branch of `parse_gemini_sse`:

warning: this if statement can be collapsed
  --> crates/mofa-foundation/src/llm/google.rs:450:25

Root Cause

The end-of-stream arm of the SSE parser had three levels of nested
`if` / `if let` / `if` guards:

if !buf.trim().is_empty() {
    if let Some(json_str) = buf.trim().strip_prefix("data: ") {
        if json_str.trim() != "[DONE]" {
            if let Ok(chunk) = serde_json::from_str::<...>(json_str) { ... }
        }
    }
}

Fix

Collapse the outer three guards into a single flat chain using
`strip_prefix` + `.filter()` so there is only one level of nesting
left (the inner `if let Ok(chunk)`):

if let Some(json_str) = buf
    .trim()
    .strip_prefix("data: ")
    .filter(|s| !s.trim().is_empty() && s.trim() != "[DONE]")
{
    if let Ok(chunk) = serde_json::from_str::<GeminiStreamChunk>(json_str) { ... }
}

Semantics are identical: the filter checks both the non-empty guard and
the [DONE] sentinel in one step.

Verification

cargo clippy -p mofa-foundation -- -D warnings
cargo test   -p mofa-foundation

Both pass clean.

Checklist

• Clippy warning eliminated
• Logic unchanged (covered by existing SSE round-trip tests)
• Fixes chore: Fix clippy::collapsible_if warnings in Gemini SSE parser #1295