fix: bump swc_core version and fix compat with swc_common

[ErnestM1234]

Why

• version 16.1 of nextjs upgrades to version 49.0.0 of swc_core breaking the swc plugin. We disabled this earlier as a temporary fix. This PR follows up on the temporary fix by adding compatibility.

What

• compatibility fixes (swc_core 49.0.0 has a few API changes)
• deprecate plugin for nextjs versions earlier than 16.1

Greptile Summary

This PR upgrades the SWC plugin to support Next.js 16.1+ by migrating to swc_core version 49.0.0 and deprecating support for older Next.js versions.

Key changes:

• Upgraded swc_core from older version to 49.0.0 with forced exact swc_common version 18.0.0
• Updated SWC_PLUGIN_SUPPORT constant from 15.2.0 to 16.1.0, effectively deprecating the plugin for Next.js < 16.1
• Migrated Rust code to use new SWC API: JSXAttrValue::Str instead of JSXAttrValue::Lit(Lit::Str(...))
• Changed string conversion from to_string() to to_string_lossy().into_owned() for proper handling of non-UTF8 sequences
• Added compatibility warning message when plugin is disabled for older Next.js versions
• Updated test code to match new API with .into() conversions for Atom values

Issue found:

• Missing template variable interpolation in swcPluginCompatibilityChangeWarning - the constant reference ${SWC_PLUGIN_SUPPORT} is not being replaced with the actual version value

Confidence Score: 4/5

• This PR is mostly safe to merge with one logic error that needs fixing
• The SWC version upgrade and API compatibility changes are well-implemented across Rust code. All string handling, JSX attribute extraction, and test updates follow the new API correctly. However, there's a critical bug in createErrors.ts where the warning message contains ${SWC_PLUGIN_SUPPORT} that won't be interpolated at runtime, showing the literal string instead of version "16.1.0". This needs to be fixed before merge.
• packages/next/src/errors/createErrors.ts requires a fix for the template string interpolation bug

Important Files Changed

Filename	Overview
packages/next/swc-plugin/Cargo.toml	Upgraded swc_core to 49.0.0 with forced exact swc_common version 18.0.0 for Next.js 16.1+ compatibility
packages/next/src/plugin/constants.ts	Updated SWC_PLUGIN_SUPPORT from 15.2.0 to 16.1.0, deprecating plugin for older Next.js versions
packages/next/src/errors/createErrors.ts	Added swcPluginCompatibilityChangeWarning but missing SWC_PLUGIN_SUPPORT import
packages/next/swc-plugin/src/visitor/jsx_utils.rs	Updated string handling to use to_string_lossy().into_owned() and changed JSX attribute API from JSXAttrValue::Lit to JSXAttrValue::Str
packages/next/swc-plugin/src/ast/utilities.rs	Updated JSX string attribute extraction to match new SWC API (JSXAttrValue::Str instead of JSXAttrValue::Lit)

Sequence Diagram

sequenceDiagram
    participant User as Developer
    participant NextConfig as withGT Config
    participant Validator as validateCompiler
    participant SWCPlugin as SWC Plugin (Rust)
    participant NextJS as Next.js 16.1+
    
    User->>NextConfig: Initialize gt-next
    NextConfig->>Validator: Check compiler compatibility
    
    alt Next.js < 16.1.0
        Validator->>Validator: swcPluginCompatible = false
        Validator->>User: Warn: SWC plugin disabled for Next.js < 16.1
        Validator->>NextConfig: Set compiler type = 'none'
    else Next.js >= 16.1.0
        Validator->>Validator: swcPluginCompatible = true
        Validator->>NextConfig: Keep compiler type = 'swc'
        NextConfig->>SWCPlugin: Load plugin with swc_core 49.0.0
        SWCPlugin->>NextJS: Transform JSX/code using new API
        Note over SWCPlugin,NextJS: Uses JSXAttrValue::Str<br/>instead of JSXAttrValue::Lit<br/>to_string_lossy() for strings
        NextJS->>User: Build succeeds
    end

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