docs(v2): add Example_ functions for pkg.go.dev discoverability

What

Adds example_test.go files to 7 v2 packages, each containing Example_* functions.

Why

Example_* functions are rendered as interactive code samples on pkg.go.dev and are the primary discoverability mechanism for Go libraries. Without them, consumers must read godoc comments and READMEs separately. With them, pkg.go.dev shows runnable snippets inline next to each type and function.

Packages covered

Package	Examples added
app	Example, ExampleNewWithConfig, ExampleApp_Register (incl. custom Component impl)
log	Example, ExampleNewWithConfig, ExampleWithFields, ExampleError
httpserver	Example, ExampleServer_AddReadinessCheck, ExampleServer_ServeHTTP
secret	Example, ExampleSecret_String, ExampleClient_Get, ExampleNewFileProvider
httpclient	Example, ExampleClient_DoWithRetry, ExampleNewTransport, ExampleDoer, ExampleClient_Post
postgres	Example, ExampleNewWithConfig
fields	Example

Testing

Examples with deterministic output use // Output: and are verified by go test. Examples involving network I/O (app lifecycle, postgres, tracing) are compiled but not executed per Go spec.