[srp-client] use proportional jitter for retry timer

[abtink]

This change updates the jitter calculation for the SRP client's retry mechanism.

Previously, a fixed jitter value was used. This could lead to synchronized retries from multiple clients, especially as the retry interval grows.

The new implementation calculates the jitter as a fraction of the current retry interval (1/5th), ensuring that the jitter scales with the wait time. This helps to better decorrelate retries from different clients.

A new constant kRetryJitterDivisor is introduced for this calculation. The jitter is clamped to a minimum value given by kRetryIntervalJitter.

[abtink]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a proportional jitter to the SRP client's retry mechanism, which is a good improvement to help decorrelate retries from multiple clients. The implementation is correct. I've added one suggestion to make the code for jitter calculation more concise and robust by initializing the variable at declaration.

[github-actions]

Merging #12044 into main

name	branch	text	data	bss	total
ot-cli-ftd	64e064f	480776	860	66580	548216
	73631de	480792	860	66580	548232
	+/-	+16	+0	+0	+16
ot-ncp-ftd	64e064f	446700	764	61824	509288
	73631de	446732	764	61824	509320
	+/-	+32	+0	+0	+32
ot-cli-mtd	64e064f	372936	764	51052	424752
	73631de	372968	764	51052	424784
	+/-	+32	+0	+0	+32
ot-ncp-mtd	64e064f	353900	764	46320	400984
	73631de	353932	764	46320	401016
	+/-	+32	+0	+0	+32
ot-cli-ftd-br	64e064f	584200	868	136412	721480
	73631de	584232	868	136412	721512
	+/-	+32	+0	+0	+32
ot-rcp	64e064f	63136	568	20804	84508
	73631de	63136	568	20804	84508
	+/-	+0	+0	+0	+0

Library files

name	branch	text	data	bss	total
libopenthread-ftd.a	64e064f	247637	95	40358	288090
	73631de	247665	95	40358	288118
	+/-	+28	+0	+0	+28
libopenthread-cli-ftd.a	64e064f	61459	0	8083	69542
	73631de	61459	0	8083	69542
	+/-	+0	+0	+0	+0
libopenthread-ncp-ftd.a	64e064f	33581	0	5948	39529
	73631de	33581	0	5948	39529
	+/-	+0	+0	+0	+0
libopenthread-mtd.a	64e064f	165719	0	24854	190573
	73631de	165747	0	24854	190601
	+/-	+28	+0	+0	+28
libopenthread-cli-mtd.a	64e064f	41414	0	8059	49473
	73631de	41414	0	8059	49473
	+/-	+0	+0	+0	+0
libopenthread-ncp-mtd.a	64e064f	25805	0	5948	31753
	73631de	25805	0	5948	31753
	+/-	+0	+0	+0	+0
libopenthread-ftd-br.a	64e064f	352431	100	110158	462689
	73631de	352459	100	110158	462717
	+/-	+28	+0	+0	+28
libopenthread-cli-ftd-br.a	64e064f	79303	0	8115	87418
	73631de	79303	0	8115	87418
	+/-	+0	+0	+0	+0
libopenthread-rcp.a	64e064f	9944	0	5060	15004
	73631de	9944	0	5060	15004
	+/-	+0	+0	+0	+0
libopenthread-radio.a	64e064f	19411	0	238	19649
	73631de	19411	0	238	19649
	+/-	+0	+0	+0	+0

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15%. Comparing base (64e064f) to head (301c7cb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/net/srp_client.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12044      +/-   ##
==========================================
+ Coverage   75.88%   77.15%   +1.26%     
==========================================
  Files         667      666       -1     
  Lines       97424    94615    -2809     
==========================================
- Hits        73932    72998     -934     
+ Misses      23492    21617    -1875     

Files with missing lines	Coverage Δ
src/core/net/srp_client.hpp	93.65% <ø> (ø)
src/core/net/srp_client.cpp	93.24% <0.00%> (+0.62%)	⬆️

... and 239 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull Request Overview

Switch SRP client retry jitter from a fixed value to a proportion of the current retry interval to better decorrelate client retries.

• Introduce kRetryJitterDivisor (1/5) and compute jitter as retry_interval / kRetryJitterDivisor, with a minimum clamp to kRetryIntervalJitter.
• Apply the computed jitter in mTimer.Start(Random::NonCrypto::AddJitter(...)).

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/core/net/srp_client.hpp	Adds kRetryJitterDivisor constant to control proportional jitter.
src/core/net/srp_client.cpp	Computes jitter proportionally to current retry interval and uses it when starting the retry timer.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}