feat(provider): add native Google Gemini API adapter (Phase 4)

Adds internal/provider/google, the second native cloud adapter (following
internal/provider/anthropic's pattern) on top of Phase 1's provider-neutral
tool-use loop, wired to a Docker-sandboxed NativeRunner under agent.type:
"google" -- a separate execution path and budget bucket from the existing
CLI-subprocess "gemini" ContainerRunner, which is untouched.

Wire-format research (the highest-risk part of this adapter): Gemini's
multi-turn function-calling shape was resolved by cross-referencing the
REST API reference's own generateContent example against the go-genai SDK's
struct tags on GitHub -- both agree on functionCall/functionResponse parts
keyed by "name" (with an optional "id" for round-tripping ToolCall.ID),
with the response fed back inside a "user"-role Content (Gemini has no
tool/function role, mirroring Anthropic's lack of one). A separate fetched
source (the function-calling guide page) was deliberately discarded as a
reference for this shape -- it documents a different, newer "Interactions
API" whose call_id/type:"function_result" structure doesn't fit the
contents/parts/candidates shape used everywhere else.

- internal/provider/google: request/response translation, systemInstruction
  handling, role mapping (assistant->model, tool-results->user role),
  per-model-prefix pricing table (2.5 Pro/Flash/Flash-Lite, 2.0, 1.5 tiers)
- internal/retry: IsRateLimitError additively extended for RESOURCE_EXHAUSTED
- internal/config: RunnersConfig.Google/GoogleEnabled()
- internal/cli/serve.go, run.go: runners["google"] construction mirroring
  the Anthropic wiring exactly (Docker sandbox default)
- docs/api-keys-setup.md: Google marked wired-up, budget-bucket/disable/
  verify guidance added matching the Anthropic section

go build/vet/test -race all pass. No live Gemini API key available in this
environment; verified via fake-httptest-server adapter tests (plain text,
tool-use round-trip, multi-turn tool-result, rate-limit error matching) plus
a Pool/NativeRunner routing test. Live E2E is a follow-up once a key is
configured, same as Phase 2's Anthropic adapter.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

1 files changed, 22 insertions, 5 deletions

diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index f71b9cc..4167ff7 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -71,11 +71,14 @@ func TestRunnersConfig_DefaultsAllEnabled(t *testing.T) {
 	if !r.AnthropicEnabled() {
 		t.Error("anthropic should be enabled by default (nil)")
 	}
+	if !r.GoogleEnabled() {
+		t.Error("google should be enabled by default (nil)")
+	}
 }
 
 func TestRunnersConfig_ExplicitFalseDisables(t *testing.T) {
 	f := false
-	r := RunnersConfig{Claude: &f, Gemini: &f, Local: &f, Container: &f, Anthropic: &f}
+	r := RunnersConfig{Claude: &f, Gemini: &f, Local: &f, Container: &f, Anthropic: &f, Google: &f}
 	if r.ClaudeEnabled() {
 		t.Error("explicit false should disable claude")
 	}
@@ -91,12 +94,15 @@ func TestRunnersConfig_ExplicitFalseDisables(t *testing.T) {
 	if r.AnthropicEnabled() {
 		t.Error("explicit false should disable anthropic")
 	}
+	if r.GoogleEnabled() {
+		t.Error("explicit false should disable google")
+	}
 }
 
 func TestRunnersConfig_ExplicitTrueEnables(t *testing.T) {
 	tr := true
-	r := RunnersConfig{Claude: &tr, Gemini: &tr, Local: &tr, Container: &tr, Anthropic: &tr}
-	if !r.ClaudeEnabled() || !r.GeminiEnabled() || !r.LocalEnabled() || !r.ContainerEnabled() || !r.AnthropicEnabled() {
+	r := RunnersConfig{Claude: &tr, Gemini: &tr, Local: &tr, Container: &tr, Anthropic: &tr, Google: &tr}
+	if !r.ClaudeEnabled() || !r.GeminiEnabled() || !r.LocalEnabled() || !r.ContainerEnabled() || !r.AnthropicEnabled() || !r.GoogleEnabled() {
 		t.Error("explicit true should keep all runners enabled")
 	}
 }
@@ -185,6 +191,10 @@ timeout_seconds = 30
 [providers.anthropic]
 api_key = "sk-ant-xyz"
 default_model = "claude-sonnet-4-5"
+
+[providers.google]
+api_key = "AIza-abc"
+default_model = "gemini-2.5-flash"
 `
 	if err := os.WriteFile(path, []byte(toml), 0600); err != nil {
 		t.Fatal(err)
@@ -195,8 +205,8 @@ default_model = "claude-sonnet-4-5"
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
-	if len(cfg.Providers) != 2 {
-		t.Fatalf("expected 2 providers, got %d: %+v", len(cfg.Providers), cfg.Providers)
+	if len(cfg.Providers) != 3 {
+		t.Fatalf("expected 3 providers, got %d: %+v", len(cfg.Providers), cfg.Providers)
 	}
 	groq, ok := cfg.Providers["groq"]
 	if !ok {
@@ -213,4 +223,11 @@ default_model = "claude-sonnet-4-5"
 	if anthropic.Endpoint != "" || anthropic.APIKey != "sk-ant-xyz" || anthropic.DefaultModel != "claude-sonnet-4-5" {
 		t.Errorf("anthropic provider config: %+v", anthropic)
 	}
+	google, ok := cfg.Providers["google"]
+	if !ok {
+		t.Fatal("expected [providers.google] table")
+	}
+	if google.Endpoint != "" || google.APIKey != "AIza-abc" || google.DefaultModel != "gemini-2.5-flash" {
+		t.Errorf("google provider config: %+v", google)
+	}
 }