Add sessions with dynamic instructions

[mattt]

This PR is the last of three that bring in #273 by @qoli. It adds LanguageModelSession.init(model:dynamicInstructions:history:), following Foundation Models 27. The session evaluates the dynamic instructions before every request to the model, including the request that continues a response after tool calls, and sends the resolved instructions and tools in that request's context (#292). The resolved instructions never become part of the session's transcript.

On OS 27, SystemLanguageModel and the other Foundation Models adapters pass the dynamic instructions to Foundation Models directly. On OS 26, SystemLanguageModel throws the new SystemLanguageModel.Error.dynamicInstructionsUnavailable, because Foundation Models there can't resolve them again for the request after tool calls.

On top of @qoli's work, this PR:

• Leaves instructions entries out of the history, so the dynamic instructions are the only instructions
• Documents that session.tools and session.instructions are empty for these sessions
• Replaces the decodingFailure thrown before OS 27 with the new error

[Copilot]

Copilot review overview

🟡 Changes recommended

The documented error contract is inaccurate for tvOS, and the new unsupported-platform branch lacks direct coverage.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Add platform-gated tests for dynamicInstructionsUnavailable · New
• Document the persistent tvOS limitation for dynamic instructions · New

What changed in this PR

Adds per-request dynamic instructions and tools to language model sessions.

Changes:

• Resolves dynamic instructions before requests and tool continuations.
• Bridges dynamic sessions to Foundation Models 27.
• Adds documentation, compatibility checks, and behavioral tests.

File	Description
Sources/​AnyLanguageModel/​LanguageModelSession.swift	Adds dynamic session initialization and request-context resolution.
Sources/​AnyLanguageModel/​Models/​SystemLanguageModel.swift	Adds native Foundation Models bridging and unsupported-platform errors.
README.md	Documents dynamic instructions.
Tests/​AnyLanguageModelTests/​DynamicInstructionsTests.swift	Tests reevaluation, history, tools, failures, and cancellation.
Tests/​AnyLanguageModelTests/​APICompatibilityFoundationModelsTests.swift	Checks Foundation Models API compatibility.
Tests/​AnyLanguageModelTests/​APICompatibilityAnyLanguageModelTests.swift	Checks AnyLanguageModel API compatibility.

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[Copilot]

Copilot review overview

🔵 Needs a closer look

The OS 27 native integration requires final human validation, and its compiler requirements are currently documented inaccurately.

Review effort: Balanced
Findings: 1

Open (1)

• Add platform-gated tests for dynamicInstructionsUnavailable

Resolved since last review (1)

• Document the persistent tvOS limitation for dynamic instructions

Previously missed (2)

In code that hasn't changed since last review

Document Swift 6.3 fallback behavior on OS 27

README.md:488

The fallback is also selected on OS 27 when the package is compiled with Swift 6.3, because the native branch is excluded by compiler(>=6.4). Document that case; otherwise an OS 27 user can unexpectedly receive the stated error despite satisfying this note.

Clarify Swift 6.3 toolchain requirement on OS 27

Sources/​AnyLanguageModel/​Models/​SystemLanguageModel.swift:367

This message is misleading for binaries built with Swift 6.3: the compiler(>=6.4) gate above makes this case throw even when the device is already on OS 27. Mention the build-toolchain requirement so users on OS 27 know that upgrading only the runtime is insufficient.