Improve error message when current platform is not in lockfile

[55728]

When bundle install fails because the current platform is not included in the lockfile, the existing error message is confusing — it says it cannot find a gem for the locked platform, then immediately lists available gems that include the very platform it claimed to be missing.

This PR adds a hint to the error message explaining the actual issue and suggesting a fix.

Fixes #9413

Example

Before:

Could not find gem 'pg' with platform 'x86_64-linux-musl' in rubygems repository
https://rubygems.org/ or installed locally.

The source contains the following gems matching 'pg':
  * pg-0.7.9.2008.01.28
  ... (200+ entries, including pg-1.6.3-x86_64-linux-musl)

After:

Could not find gem 'pg' with platform 'x86_64-linux-musl' in rubygems repository
https://rubygems.org/ or installed locally.

The source contains the following gems matching 'pg':
  * pg-0.7.9.2008.01.28
  ... (same as before)

Your current platform (x86_64-linux) is not included in the lockfile's platforms
(x86_64-linux-musl). Add the current platform to the lockfile with
`bundle lock --add-platform x86_64-linux` and try again.

Implementation

Added a platform_mismatch_hint method in Bundler::Resolver that checks Bundler.locked_gems.platforms (the lockfile's PLATFORMS section) against Bundler.local_platform. When the current platform is not in the lockfile, the hint is appended to the existing error message.