feat(pageserver): support synthetic size calculation for invisible branches (#11335)

## Problem

ref https://github.com/neondatabase/neon/issues/11279


Imagine we have a branch with 3 snapshots A, B, and C:
```
base---+---+---+---main
        \-A \-B \-C
base=100G, base-A=1G, A-B=1G, B-C=1G, C-main=1G
```
at this point, the synthetic size should be 100+1+1+1+1=104G.

after the deletion, the structure looks like:
```
base---+---+---+
       \-A \-B \-C
```
If we simply assume main never exists, the size will be calculated as
size(A) + size(B) + size(C)=300GB, which obviously is not what the user
would expect.

The correct way to do this is to assume part of main still exists, that
is to say, set C-main=1G:
```
base---+---+---+main
       \-A \-B \-C
```
And we will get the correct synthetic size of 100G+1+1+1=103G.


## Summary of changes

* Do not generate gc cutoff point for invisible branches.
* Use the same LSN as the last branchpoint for branch end.
* Remove test_api_handler for mark_invisible.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/http/routes.rs

@@ -3774,7 +3774,7 @@ pub fn make_router(
         )
         .put(
             "/v1/tenant/:tenant_shard_id/timeline/:timeline_id/mark_invisible",
-            |r| testing_api_handler("mark timeline invisible", r, timeline_mark_invisible_handler),
+            |r| api_handler( r, timeline_mark_invisible_handler),
         )
         .put(
             "/v1/tenant/:tenant_shard_id/timeline/:timeline_id/checkpoint",