add missing strip_prefix

[Thanhphan1147]

The code to strip the ingress prefix was present when synapse was using ingress v1 but was missed when moving to the v2 lib. So this PR adds it. synapse can't handle prefix so this is needed to function properly.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Manging Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)

[amandahla]

@Thanhphan1147 can this PR be merged once integration tests finish?

[amandahla]

@Thanhphan1147 Could this PR be merged?

[yhaliaw]

It seems this is a user-affecting bug fix.
It might be good to update the docs/changelog.md.

[github-actions]

Test results for commit accc7ef

Test coverage for accc7ef

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
src/actions.py                             36      0      6      0   100%
src/auth/__init__.py                        0      0      0      0   100%
src/auth/mas.py                            86      6      2      0    93%   78-80, 97-99
src/backup.py                             175      5     20      2    96%   351-352, 421-422, 479->481, 482
src/backup_observer.py                    151     16     14      0    90%   162-165, 170-173, 210-213, 242-245
src/charm.py                              276     26     62     12    89%   179->181, 184, 258, 311, 315-316, 322-323, 341-342, 366, 373, 448-452, 455-456, 484-486, 506, 536-542, 572-573
src/charm_types.py                         30      0      0      0   100%
src/database_client.py                     57      1      8      4    92%   35, 47->exit, 69->exit, 88->98
src/database_observer.py                   49      4      2      0    92%   61-64
src/exceptions.py                           3      0      0      0   100%
src/matrix_auth_observer.py                59      5      6      0    92%   133-137
src/media_observer.py                      45      4      2      1    89%   60-62, 81
src/observability.py                       13      0      0      0   100%
src/pebble.py                             221     38     50     12    82%   158->exit, 169-173, 218-219, 239-240, 258-261, 322->327, 332-333, 345-346, 348-349, 380, 382, 384, 386, 388, 395-398, 426, 577-593, 635-642
src/redis_observer.py                      39      3      4      0    93%   63-66
src/s3_parameters.py                       22      0      4      0   100%
src/smtp_observer.py                       61      1     14      2    96%   89, 108->113
src/state/__init__.py                       0      0      0      0   100%
src/state/charm_state.py                  144     11     34      7    90%   168, 172, 193, 218, 224, 230, 234-235, 321, 388-389
src/state/mas.py                           79      8      6      3    87%   154, 160-161, 188-190, 205, 225
src/state/validation.py                    36      3      2      0    92%   110-112
src/synapse/__init__.py                     2      0      0      0   100%
src/synapse/workload.py                   152      7     30      2    93%   370-373, 404-405, 453
src/synapse/workload_configuration.py     149     26     34     12    79%   90->exit, 94-95, 143-144, 173, 193-194, 226-227, 260, 269-270, 285, 290-291, 312-313, 332->337, 338, 356->358, 368-369, 397, 405->407, 407->409, 414-415, 435->442, 445, 465-466
src/user.py                                23      5      2      1    76%   27-29, 56, 73
-----------------------------------------------------------------------------------
TOTAL                                    1908    169    302     58    90%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:01
Run started:2025-04-22 13:01:14.484518

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 9902
  Total lines skipped (#nosec): 6
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):