refactor: introduce Source trait for random generator

[peter-jerry-ye]

Closes #1589

[peter-jerry-ye-code-review]

Missing type constraint on `Source` implementation

Category
Correctness
Code Snippet
impl Source for @random_source.ChaCha8 with next(self : @random_source.ChaCha8) -> UInt64
Recommendation
Add a check to ensure source implementation can generate values in the expected range
Reasoning
The Source trait implementation should guarantee that generated numbers meet certain quality requirements. Without constraints, implementations could provide poor quality or biased random numbers.

Deprecated function still used in documentation example

Category
Maintainability
Code Snippet
test {
let r = @random.new()
...
}
Recommendation
Update the example in README.mbt.md to use the new constructor style:

let r = @random.Rand::new(@random.chacha8())

Reasoning
Documentation should reflect current best practices and avoid showing deprecated APIs to prevent new code from using outdated patterns.

Missing documentation for trait methods

Category
Maintainability
Code Snippet
pub(open) trait Source {
next(Self) -> UInt64
}
Recommendation
Add documentation for the next method explaining its contract:

pub(open) trait Source {
  ///| Generates the next 64-bit unsigned random integer
  next(Self) -> UInt64
}

Reasoning
Trait method documentation is crucial for implementers to understand the requirements and guarantees they need to provide.

[coveralls]

Pull Request Test Coverage Report for Build 6209

Details

• 4 of 7 (57.14%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 91.52%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
random/random.mbt	4	7	57.14%

Totals
Change from base Build 6202:	-0.05%
Covered Lines:	5634
Relevant Lines:	6156

---

💛 - Coveralls

[peter-jerry-ye]

add a default value