Improve error message for || (or) in let chains

[Kivooeo]

Description

This PR improves the error message when using || in an if let chain expression, addressing #140263.

Changes

1. Creates a dedicated error message specifically for || usage in let chains
2. Points the primary span directly at the || operator
3. Removes confusing secondary notes about "let statements" and unsupported contexts
4. Adds UI tests verifying the new error message and valid cases

Before

error: expected expression, found let statement
 --> src/main.rs:2:8
  |
2 |     if let true = true || false {}
  |        ^^^^^^^^^^^^^^^
  |
  = note: only supported directly in conditions of if and while expressions
note: || operators are not supported in let chain expressions
 --> src/main.rs:2:24
  |
2 |     if let true = true || false {}
  |       

After

error: `||` operators are not supported in let chain conditions
 --> src/main.rs:2:24
  |
2 |     if let true = true || false {}
  |                        ^^

Implementation details

1. Added new OrInLetChain diagnostic in errors.rs

2. Modified CondChecker in expr.rs to prioritize the || error

3. Updated fluent message definitions to use clearer wording

Related issue
Fixes #140263

cc @ehuss (issue author)

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[Kivooeo]

@est31 if you interested in this pr i would love to see you as reviewer, also seems like there was an update or something so got conflict :(
I will resolve it tomorrow but for now you can see tests and how it works, thanks!
I also saw your recommendation about change "expression" wording to "condition" I will fix it as well tomorrow

[est31]

@bors r+ rollup

[bors]

📌 Commit f072d30 has been approved by est31

It is now in the queue for this repository.