chore(main): release 2.9.0

[release-please]

🤖 I have created a release beep boop

2.9.0 (2026-06-12)

Features

• Add ServiceTier to UsageMetadata (45b4963)
• Gemma 4 local tokenizer support (ca97c58)
• interaction-api: Add model field and CodeMenderAgentData response message (4750c75)
• interaction-api: Add CodeMenderAgentConfig to the Interactions API AgentInteraction proto (3900799)
• interaction-api: Add presence_penalty, frequency_penalty, and cached_content to models.proto (05f16fe)
• interaction-api: Rename usage to total_usage in StreamMetadata. (7c331c6)

Bug Fixes

• Add fallback for aiohttp.readline without max_line_length for backward compatibility because we still want to keep aiohttp as optional dependency (e99ab99), closes #2487
• Fix header (f8f9749)
• Keep live music API keys out of websocket urls (#2564) (c754ebf)
• Make transformers an optional dependency for local tokenizers, also add other dependencies to local-tokenizer-extras. (528926b)
• Use .model_copy() instead of deprecated .copy() (216369f)

Documentation

• Announce Automatic Function Calling (AFC) upcoming breaking change warning (4697258)
• Clarify Live API START/END_SENSITIVITY_HIGH/LOW defaults are different in Gemini Live and Gemini Enterprise Agent Platform Live API (a0ec6ab), closes #2555
• Regenerate docs for 2.8.0 (93e7ab1)

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[yyyu-google]

@jaycee-li can remove the do not merge tag Monday