Add Anthropic sampling handler and consolidate sampling examples

- Add AnthropicSamplingHandler in server/sampling/anthropic.py
- Consolidate all sampling examples into examples/sampling/ with rich output
- Examples: text.py, structured_output.py, tool_use.py, server_fallback.py
- Add anthropic optional dependency

examples/sampling/server_fallback.py

@@ -0,0 +1,151 @@
+# /// script
+# dependencies = ["anthropic", "fastmcp", "rich"]
+# ///
+"""
+Server-Side Sampling Fallback Example
+
+When the CLIENT has no sampling handler, the SERVER's handler is used instead.
+
+MCP Flow with Fallback:
+1. Client calls server tool (client has NO sampling handler)
+2. Server tool calls ctx.sample()
+3. Client can't handle sampling → server's fallback handler is used
+4. Server's handler calls the LLM directly
+5. Response returns to server, then to client
+
+Run:
+    python examples/sampling/server_fallback.py
+"""
+
+import asyncio
+
+from rich.console import Console
+from rich.panel import Panel
+from rich.text import Text
+
+from fastmcp import Client, Context, FastMCP
+from fastmcp.server.sampling.anthropic import AnthropicSamplingHandler
+
+console = Console()
+
+
+# ============================================================================
+# SERVER - Note: sampling_handler is on the SERVER, not the client
+# ============================================================================
+
+
+class LoggingAnthropicHandler(AnthropicSamplingHandler):
+    async def __call__(self, messages, params, context):
+        console.print(
+            "      [bold blue]⚡ FALLBACK[/] Server's handler calling Claude...",
+            highlight=False,
+        )
+        result = await super().__call__(messages, params, context)
+        console.print(
+            "      [bold blue]⚡ FALLBACK[/] Response received", highlight=False
+        )
+        return result
+
+
+mcp = FastMCP(
+    "Server with Fallback",
+    sampling_handler=LoggingAnthropicHandler(default_model="claude-sonnet-4-20250514"),
+)
+
+
+@mcp.tool
+async def get_fun_fact(topic: str, ctx: Context) -> str:
+    """Get a fun fact about a topic."""
+    console.print("   [bold yellow]📦 SERVER[/] Tool 'get_fun_fact' called")
+    console.print(
+        "   [bold yellow]📦 SERVER[/] Requesting sampling (client has no handler!)"
+    )
+
+    result = await ctx.sample(
+        messages=f"Tell me one fun fact about: {topic}",
+        system_prompt="Share a fascinating fact in 1-2 sentences.",
+    )
+
+    console.print("   [bold yellow]📦 SERVER[/] Got response via fallback handler")
+    return result.text  # type: ignore[return-value]
+
+
+@mcp.tool
+async def translate(text: str, language: str, ctx: Context) -> str:
+    """Translate text to another language."""
+    console.print("   [bold yellow]📦 SERVER[/] Tool 'translate' called")
+    console.print(
+        "   [bold yellow]📦 SERVER[/] Requesting sampling (client has no handler!)"
+    )
+
+    result = await ctx.sample(
+        messages=f"Translate to {language}: {text}",
+        system_prompt="Provide only the translation.",
+    )
+
+    console.print("   [bold yellow]📦 SERVER[/] Got response via fallback handler")
+    return result.text  # type: ignore[return-value]
+
+
+# ============================================================================
+# CLIENT - Note: NO sampling_handler provided!
+# ============================================================================
+
+
+async def main():
+    console.print()
+    console.print(
+        Panel.fit(
+            "[bold]Server Fallback Example[/]\n\n"
+            "The [green]CLIENT[/] has [bold red]no sampling handler[/].\n"
+            "The [yellow]SERVER's[/] fallback handler is used instead.",
+            border_style="bright_black",
+        )
+    )
+    console.print()
+
+    # IMPORTANT: No sampling_handler!
+    async with Client(mcp) as client:
+        console.rule(style="dim")
+        console.print(
+            "[bold green]🖥️  CLIENT[/] Calling 'get_fun_fact' [dim](no sampling handler!)[/]"
+        )
+        console.print()
+
+        result = await client.call_tool("get_fun_fact", {"topic": "octopuses"})
+
+        console.print()
+        console.print("[bold green]🖥️  CLIENT[/] Result:")
+        console.print(Panel(result.data, border_style="green"))
+        console.print()
+
+        console.rule(style="dim")
+        console.print(
+            "[bold green]🖥️  CLIENT[/] Calling 'translate' [dim](no sampling handler!)[/]"
+        )
+        console.print()
+
+        result = await client.call_tool(
+            "translate", {"text": "Hello, how are you?", "language": "Spanish"}
+        )
+
+        console.print()
+        console.print("[bold green]🖥️  CLIENT[/] Result:")
+        console.print(Panel(result.data, border_style="green"))
+        console.print()
+
+    # Summary
+    summary = Text()
+    summary.append("Key concept: ", style="bold")
+    summary.append(
+        "When the client lacks sampling support, the server's\n"
+        "fallback handler steps in. This lets servers guarantee\n"
+        "sampling works regardless of client capabilities."
+    )
+
+    console.print(Panel(summary, border_style="bright_black"))
+    console.print()
+
+
+if __name__ == "__main__":
+    asyncio.run(main())