fix(mongodb): hide topology.pool_size and fix it at 1

[thalesmg]

Fixes https://emqx.atlassian.net/browse/EMQX-10408

From an old conversation with @kjellwinblad:

There are 3 pool_sizes

• The buffer workers pool size, just exposed here: feat(mongo): expose buffer worker opts to the bridge frontend (5.0) #9742
• The topology.pool_size, which controls the pool size for the poolboy_pool in Kjell's
  diagram (on mongodb's side).
• The pool_size from emqx_connector_mongo:mongo_fields that controls the ecpool pool
  size (on EMQX's side).
  So we actually want to set topology.pool_size = 1 and hide it from users.

Summary

🤖 Generated by Copilot at 40aecf4

Hide pool_size option and set it to 1 for MongoDB plugin. This avoids confusion and improves compatibility with the MongoDB driver.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• [na] Added tests for the changes
• [na] Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• [na] If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• [na] If changed package build workflow, pass this action (manual trigger)
• [na] Change log has been added to changes/ dir for user-facing artifacts update

[thalesmg]

Will require changes in the frontend. Attempt PR: emqx/emqx-dashboard5#1568

[coveralls]

Pull Request Test Coverage Report for Build 5423344081

• 4 of 7 (57.14%) changed or added relevant lines in 1 file are covered.
• 29 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.001%) to 81.937%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_mongodb/src/emqx_mongodb.erl	4	7	57.14%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_gateway_mqttsn/src/emqx_mqttsn_frame.erl	1	64.04%
apps/emqx/src/emqx_channel.erl	1	87.41%
apps/emqx/src/emqx_schema.erl	1	87.5%
apps/emqx/src/emqx_session.erl	1	87.65%
apps/emqx/src/emqx_ws_connection.erl	1	76.37%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	3	93.32%
apps/emqx_oracle/src/emqx_oracle.erl	4	80.0%
apps/emqx/src/emqx_reason_codes.erl	17	83.82%

Totals
Change from base Build 5421999361:	0.001%
Covered Lines:	30682
Relevant Lines:	37446

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💛 - Coveralls