chore: Update Version String (#2380)

## Description

This PR modifies the User Agent (version string) generation to better
accommodate the recent support for combining prebuilt and custom tools.

The design for this update is as so:
Single Prebuilt: <version+metadata>+prebuilt.bigquery
Multiple prebuilt: <version+metadata>+prebuilt.bigquery+prebuilt.alloydb
Prebuilt + Custom:  <version+metadata>+custom.bigquery
Multiple prebuilt + custom:
<version+metadata>+custom.bigquery+custom.alloydb

Versioning Logic:
- Single Prebuilt: Appends +prebuilt.<name>
  Example: ```<version>+prebuilt.bigquery```

- Multiple Prebuilt: Appends a tag for each prebuilt config.
  Example: ```<version>+prebuilt.bigquery+prebuilt.alloydb```

- Prebuilt + Custom: If custom tools are detected, the tag for all
prebuilt tools switches to custom.
  Example: ```<version>+custom.bigquery```

- Multiple Prebuilt + Custom: All prebuilt tools receive the custom tag.
  Example: ```<version>+custom.bigquery+custom.alloydb```

## PR Checklist

> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [ ] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

README.md

@@ -107,7 +107,7 @@ redeploying your application.
 
 ## Getting Started
 
-### (Non-production) Running Toolbox
+### Quickstart: Running Toolbox using NPX
 
 You can run Toolbox directly with a [configuration file](#configuration):
 

cmd/root.go

@@ -997,9 +997,6 @@ func run(cmd *Command) error {
 				return err
 			}
 
-			// Update version string
-			cmd.cfg.Version += "+prebuilt." + configName
-
 			// Parse into ToolsFile struct
 			parsed, err := parseToolsFile(ctx, buf)
 			if err != nil {
@@ -1066,6 +1063,18 @@ func run(cmd *Command) error {
 		allToolsFiles = append(allToolsFiles, customTools)
 	}
 
+	// Modify version string based on loaded configurations
+	if len(cmd.prebuiltConfigs) > 0 {
+		tag := "prebuilt"
+		if isCustomConfigured {
+			tag = "custom"
+		}
+		// cmd.prebuiltConfigs is already sorted above
+		for _, configName := range cmd.prebuiltConfigs {
+			cmd.cfg.Version += fmt.Sprintf("+%s.%s", tag, configName)
+		}
+	}
+
 	// Merge Everything
 	// This will error if custom tools collide with prebuilt tools
 	finalToolsFile, err := mergeToolsFiles(allToolsFiles...)

docs/en/getting-started/introduction/_index.md

@@ -77,7 +77,7 @@ redeploying your application.
 
 ## Getting Started
 
-### (Non-production) Running Toolbox
+### Quickstart: Running Toolbox using NPX 
 
 You can run Toolbox directly with a [configuration file](../configure.md):