fix(create_project): handle gitea generate-async branch race (#42)

Live e2e surfaced a race unit tests couldn't (mocks are instant): gitea's
/generate returns and serves reads before the branch ref is writable, so the
first content writes 404 "branch does not exist" for a beat — aborting the
whole substitution pass. Add waitForBranch (poll BranchExists after generate)
+ upsertRetry (retry writes on the transient not-found). Test fake now serves
the branch readiness probe. Refs #42.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/create_project_from_template.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+	"time"
 
 	"gitea.d-ma.be/mathias/gitea-mcp/internal/allowlist"
 	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
@@ -150,6 +151,12 @@ func (t *CreateProjectFromTemplate) Call(ctx context.Context, raw json.RawMessag
 	}
 	result.DefaultBranch = branch
 
+	// Wait for the generated branch to become writable. gitea's /generate is
+	// async: it returns (and serves reads) before the branch ref is committed, so
+	// the first writes 404 "branch does not exist" for a beat. Poll until the
+	// branch is present.
+	t.waitForBranch(ctx, args.Owner, args.Name, branch)
+
 	// Substitute across the WHOLE tree: content in every blob, plus a path rename
 	// for any file whose path carries a placeholder (e.g. cmd/__PROJECT_NAME__/main.go).
 	// A fixed known-files list can't rename directories or cover every templated
@@ -185,6 +192,42 @@ func (t *CreateProjectFromTemplate) Call(ctx context.Context, raw json.RawMessag
 	return textOK(result)
 }
 
+// waitForBranch polls until the freshly-generated branch is present, or a short
+// budget elapses. Best-effort — upsertRetry covers the remaining read-vs-write
+// consistency window where the branch reads but does not yet write.
+func (t *CreateProjectFromTemplate) waitForBranch(ctx context.Context, owner, name, branch string) {
+	for i := 0; i < 20; i++ {
+		if ok, err := t.c.BranchExists(ctx, owner, name, branch); ok && err == nil {
+			return
+		}
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(500 * time.Millisecond):
+		}
+	}
+}
+
+// upsertRetry retries UpsertFile through the brief post-generate window where the
+// branch is readable but writes still 404 "branch does not exist".
+func (t *CreateProjectFromTemplate) upsertRetry(ctx context.Context, owner, name, path string, args gitea.UpsertFileArgs) error {
+	var err error
+	for i := 0; i < 5; i++ {
+		if _, err = t.c.UpsertFile(ctx, owner, name, path, args); err == nil {
+			return nil
+		}
+		if !errors.Is(err, gitea.ErrNotFound) {
+			return err
+		}
+		select {
+		case <-ctx.Done():
+			return err
+		case <-time.After(500 * time.Millisecond):
+		}
+	}
+	return err
+}
+
 // substituteEntry substitutes placeholders in one blob. If the path carries a
 // placeholder it renames the file (write new + delete old); otherwise it rewrites
 // content in place when changed. Returns a human-readable description of what was
@@ -212,7 +255,7 @@ func (t *CreateProjectFromTemplate) substituteEntry(ctx context.Context, owner,
 	enc := base64.StdEncoding.EncodeToString([]byte(newContent))
 
 	if renamed {
-		if _, err := t.c.UpsertFile(ctx, owner, name, newPath, gitea.UpsertFileArgs{
+		if err := t.upsertRetry(ctx, owner, name, newPath, gitea.UpsertFileArgs{
 			Branch:  branch,
 			Content: enc,
 			Message: fmt.Sprintf("template: substitute + rename %s -> %s", path, newPath),
@@ -229,7 +272,7 @@ func (t *CreateProjectFromTemplate) substituteEntry(ctx context.Context, owner,
 		return path + " -> " + newPath, ""
 	}
 
-	if _, err := t.c.UpsertFile(ctx, owner, name, path, gitea.UpsertFileArgs{
+	if err := t.upsertRetry(ctx, owner, name, path, gitea.UpsertFileArgs{
 		Branch:  branch,
 		Content: enc,
 		Message: "template: substitute placeholders",