refactor(map): deprecate op-get for future migration

[peter-jerry-ye]

In the future, we will have op_get return V instead of V? and get return V?.

For other functions, if there are a 'unsafe' implementation and a 'safe', the unsafe one will be named as unsafe_xxx. Otherwise it will just named as it is.

cc @Yoorkin the json.mbt emitted two same warnings

Related #1332

[peter-jerry-ye-code-review]

Missing deprecation message in some interface files

Category
Correctness
Code Snippet
builtin/builtin.mbti:
#deprecated
op_get[K : Hash + Eq, V](Self[K, V], K) -> V?
Recommendation
Add consistent deprecation message across all files:
#deprecated("Use get instead. op_get will return V instead of Option[V] in the future.")
Reasoning
Some interface files (.mbti) only have #deprecated without the message. Adding consistent messages helps developers understand why the function is deprecated and what to use instead.

Change in json.mbt might need additional null checking

Category
Maintainability
Code Snippet
json/json.mbt:
Some(obj) => obj.get(key)
None => None
Recommendation
Consider whether additional handling is needed for the case where get returns None:
Some(obj) => match obj.get(key) {
Some(v) => Some(v)
None => None
}
None => None
Reasoning
The change from obj[key] to obj.get(key) might need explicit handling of the None case if get returns an optional value.

Consider adding migration guide in documentation

Category
Maintainability
Code Snippet
Multiple files: adding #deprecated attribute
Recommendation
Add a migration guide in documentation explaining:

1. Why op_get behavior is changing
2. Timeline for the change
3. How to migrate existing code
   Reasoning
   Since this is a significant change in API behavior (from returning V? to V), users need clear guidance on how to update their code before the change takes effect.

[coveralls]

Pull Request Test Coverage Report for Build 6576

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 92.41%

---

Totals
Change from base Build 6574:	0.01%
Covered Lines:	6136
Relevant Lines:	6640

---

💛 - Coveralls