Add off() method to Listenable event interface

[noencke]

Description

This exposes the off() method on the Listenable event interface, providing an additional way to unsubscribe from events. The current way of unsubscribing, via the deregistration function returned by on(), is still supported. Having both options available is meant to give customers options without being opinionated about the best pattern, as it depends on preference and use case.

To prevent the two strategies from interfering with each other, it has also been made illegal to register the same listener more than once for the same event. It is now documented as throwing an error rather than as undefined behavior.

This also adds a test for registering events with keys that are JS symbols and does some minor doc cleanup.

[msfluid-bot]

Code Coverage Summary

↑ packages.dds.tree.src.events:
Line Coverage Change: -1.68%    Branch Coverage Change: 1.88%

Metric Name	Baseline coverage	PR coverage	Coverage Diff
Branch Coverage	91.66%	93.54%	↑ 1.88%
Line Coverage	71.65%	69.97%	↓ -1.68%

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Baseline commit: e0e9c89
Baseline build: 305911
Happy Coding!!

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[Copilot]

Copilot reviewed 5 out of 14 changed files in this pull request and generated no suggestions.

Files not reviewed (9)

• packages/dds/tree/api-report/tree.legacy.public.api.md: Evaluated as low risk
• packages/framework/fluid-framework/api-report/fluid-framework.public.api.md: Evaluated as low risk
• packages/dds/tree/api-report/tree.beta.api.md: Evaluated as low risk
• packages/dds/tree/api-report/tree.legacy.alpha.api.md: Evaluated as low risk
• packages/framework/fluid-framework/api-report/fluid-framework.alpha.api.md: Evaluated as low risk
• packages/dds/tree/api-report/tree.public.api.md: Evaluated as low risk
• packages/dds/tree/api-report/tree.alpha.api.md: Evaluated as low risk
• packages/framework/fluid-framework/api-report/fluid-framework.legacy.public.api.md: Evaluated as low risk
• packages/framework/fluid-framework/api-report/fluid-framework.legacy.alpha.api.md: Evaluated as low risk

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[jason-ha]

Thanks for making even better test code (better examples for all)

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluidframework-docs@0.25.0 ci:linkcheck /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test ci:start 1313 linkcheck:full

1: starting server using command "npm run ci:start"
and when url "[ 'http://127.0.0.1:1313' ]" is responding with HTTP status code 200
running tests using command "npm run linkcheck:full"


> fluidframework-docs@0.25.0 ci:start
> http-server ./public --port 1313 --silent


> fluidframework-docs@0.25.0 linkcheck:full
> npm run linkcheck:fast -- --external


> fluidframework-docs@0.25.0 linkcheck:fast
> linkcheck http://localhost:1313 --skip-file skipped-urls.txt --external

Crawling...

Stats:
  443362 links
    3411 destination URLs
       2 URLs ignored
       0 warnings
       0 errors

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +633 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	464.33 KB	464.36 KB	⯅ +35 Bytes
azureClient.js	563.18 KB	563.23 KB	⯅ +49 Bytes
connectionState.js	724 Bytes	724 Bytes	■ No change
containerRuntime.js	262.48 KB	262.5 KB	⯅ +14 Bytes
fluidFramework.js	426.65 KB	426.85 KB	⯅ +208 Bytes
loader.js	134.18 KB	134.19 KB	⯅ +14 Bytes
map.js	42.71 KB	42.71 KB	⯅ +7 Bytes
matrix.js	148.36 KB	148.36 KB	⯅ +7 Bytes
odspClient.js	528.97 KB	529.02 KB	⯅ +49 Bytes
odspDriver.js	97.88 KB	97.9 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.81 KB	42.83 KB	⯅ +14 Bytes
sharedString.js	164.26 KB	164.27 KB	⯅ +7 Bytes
sharedTree.js	417.11 KB	417.3 KB	⯅ +201 Bytes
Total Size	3.37 MB	3.37 MB	⯅ +633 Bytes

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Baseline commit: e0e9c89

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